Holes in the ground come from an enormous range of sources, from thumb-sized tunnels dug by solitary bees to sprawling underground networks excavated by badgers, and from slow chemical dissolution of bedrock to the dramatic collapse of soil pipes after heavy rain. The culprits fall into three broad camps: burrowing animals (mammals, reptiles, amphibians, crabs), ground-nesting insects and other invertebrates, and purely geological or hydrological forces that need no living creature at all. Figuring out what made a particular hole usually comes down to its size, shape, location, and the soil piled around it.
Mammals That Move Serious Amounts of Earth
When most people picture a hole in the ground, they think of a mammal burrow, and for good reason. Badgers, foxes, groundhogs, moles, gophers, prairie dogs, rabbits, and wombats all excavate underground living spaces, and some of them reshape the landscape in the process. Wombats in southeastern Australia, for instance, dig extensive tunnel systems into stream banks. One study measuring soil displacement at wombat sites in agricultural land found that the volume of earth moved ranged from about 8 to 89 cubic meters per hectare, amounting to several tonnes of dirt pushed to the surface per site.1Geomorphology. The geomorphic signature of bare-nosed wombats (Vombatus ursinus) and cattle (Bos taurus) in an agricultural riparian ecosystem Those mounds of ejected soil are a dead giveaway, and wombat burrows are large enough that you could fit a small dog inside the entrance.
European badgers and foxes leave their mark differently. Their setts and dens bring deep soil to the surface, and that deeper material tends to be less acidic and richer in minerals like calcium, magnesium, and potassium than the topsoil around it. Badgers have a bigger effect than foxes because they displace more material and disturb wider patches of ground.2Ecological Research. Burrowing by badgers (Meles meles) and foxes (Vulpes vulpes) changes soil conditions and vegetation in a European temperate forest If you find a hole in a European woodland surrounded by a fan of pale, chalky-looking soil that contrasts with the dark leaf litter nearby, a badger is a strong suspect.
An interesting pattern across mammalian burrowers is that bigger animals dig bigger individual burrows, but smaller species collectively move more soil per unit of time across a landscape. A colony of pocket gophers or prairie dogs, each animal tiny compared to a wombat, can churn through vast areas simply because there are so many of them working simultaneously.3Progress in Physical Geography: Earth and Environment. Soil movement by burrowing mammals So a yard pockmarked with dozens of small holes is often the work of a numerous small species, not one large digger. Mole tunnels, vole runs, chipmunk burrows, and gopher holes all fit this pattern.
Ground-Nesting Insects and Other Invertebrates
Insects are responsible for far more holes in the ground than most people realize. The majority of the world’s bee species are solitary and nest in soil, not in hives. These ground-nesting bees dig narrow tunnels into bare or sparsely vegetated earth, leaving small, cleanly cut holes about the width of a pencil, often surrounded by a tiny cone of excavated soil. Only one solitary ground-nesting bee species has ever been managed commercially for crop pollination, but many others are effective pollinators that quietly nest in lawns, garden beds, and field margins.4PubMed. Solitary Ground-Nesting Bees: Opportunities and Challenges for Crop Pollination Management If you notice a cluster of small, perfectly round holes appearing in dry, sandy soil during spring, ground-nesting bees are a likely explanation. They are not aggressive and rarely sting.
Ants build some of the most architecturally complex underground structures of any organism. One species in Brazil constructs nests that begin with a wide entrance hole measuring up to about 12 centimeters across, functioning almost like a pitfall trap for prey. Below the surface, the nest descends as a vertical shaft with progressively narrower channels connecting dish-shaped chambers. Some nests reach depths of more than five meters and contain up to 14 chambers.5Insect Science. The nest architecture of the ant, Pheidole oxyops Forel, 1908 (Hymenoptera: Formicidae) Other ant species dig shallower but still impressive nests. Lab experiments with colonies of rock ants showed that groups of 100 workers could excavate nests between about 5 and 23 centimeters deep in just a week, and the ants dug deeper when the surface temperature was warmer.6Scientific Reports. Ant nest architecture is shaped by local adaptation and plastic response to temperature That temperature sensitivity helps explain why ant mounds and entrance holes seem to appear in greater numbers during hot, dry spells.
Cicadas are another insect worth knowing about, especially if you live in eastern North America. After spending years underground feeding on root sap, periodical cicada nymphs dig exit tunnels to the surface when they are ready to emerge as adults. These mass emergences leave the ground peppered with finger-width holes, sometimes hundreds per square meter in a heavily infested area. The nymphs also build small turrets of mud around the openings.7Soil Science Society of America Journal. Soil Disturbance by the Emergence of Periodical Cicadas Cicada holes are easy to identify because they appear suddenly and in huge numbers, usually in late spring, and disappear again as the soil settles.
Earthworms deserve a mention here too. They do not leave conspicuous surface holes the way bees or ants do, but their constant tunneling creates a network of channels through the soil that profoundly affects how water and air move underground. The pore systems around earthworm burrows differ from those around plant roots, and the connectivity of these channels matters for drainage and aeration throughout the soil profile.8European Journal of Soil Science. Millimetre scale aeration of the rhizosphere and drilosphere
Reptiles and Amphibians
Tortoises are among the most important reptilian burrowers. The gopher tortoise of the southeastern United States digs deep burrows that can extend several meters underground. These burrows serve as shelter not just for the tortoise but for a remarkable number of other species: more than 300 invertebrate and 60 vertebrate species have been documented using gopher tortoise burrows for refuge, foraging, or nesting.9Ichthyology & Herpetology. Gopher Tortoise (Gopherus polyphemus) Vertebrate Burrow Commensals within a Private, Working Forest Landscape The burrows function as underground apartment buildings for the local ecosystem, and even abandoned or inactive tortoise burrows continue to provide resources to other animals.9Ichthyology & Herpetology. Gopher Tortoise (Gopherus polyphemus) Vertebrate Burrow Commensals within a Private, Working Forest Landscape
Frogs are not typically thought of as burrowers, but many species dig into the ground to survive harsh conditions. Some use their hind legs to shimmy backward into soft soil; others go headfirst. One South American frog digs using its forelimbs and bends its head downward at nearly a right angle to its body while burrowing. The process has distinct stages: the frog first pushes its head into the soil, then works its body underground, and finally constructs a small chamber where it can wait out the dry season in a dormant state.10Journal of Ethology. Burrowing behavior of Dermatonotus muelleri (Anura, Microhylidae) with reference to the origin of the burrowing behavior of Anura These frog burrows are small and inconspicuous, but in tropical regions with pronounced wet and dry seasons, they are common.
Fiddler Crabs and Coastal Burrowers
If you have ever walked along a mudflat or mangrove shoreline and noticed the ground riddled with small holes, fiddler crabs are almost certainly responsible. These semi-terrestrial crustaceans are abundant on tropical, subtropical, and warm-temperate coasts worldwide, and they are prolific diggers. At least 47 species of fiddler crab build above-ground sedimentary structures at or near the openings of their burrows, including chimneys, hoods, pillars, and mud balls.11SpringerLink (Journal of Ethology). Fiddler crabs and their above-ground sedimentary structures: a review The shape and style of these structures vary by species and by sex. Males often build more elaborate structures during mating season, using them as visual signals. So if you see a small hole in coastal mud surrounded by a neat little chimney of sediment, a fiddler crab built it.
Other coastal burrowers include ghost crabs, which dig deeper tunnels on sandy beaches, and various species of shrimp that excavate complex burrow systems in tidal flats. These animals collectively turn over enormous volumes of sediment, and their holes are a normal, healthy feature of shoreline ecosystems.
Geological and Hydrological Holes
Not every hole in the ground is the work of a living creature. Some of the most dramatic ones form through purely geological and hydrological processes. Sinkholes are perhaps the best-known example. In regions underlain by soluble rock like limestone, dolomite, or gypsum, surface water and groundwater slowly dissolve the rock from within, creating underground voids. Over time, the roof of a void weakens and collapses, sometimes gradually, sometimes catastrophically. These karst landscapes, as geologists call them, produce enclosed depressions, sinking streams, and caves, all primarily driven by water dissolving rock, with mechanical erosion playing a secondary role.12Watershed Ecology and the Environment. Karst topography: Formation, processes, characteristics, landforms, degradation and restoration: A systematic review
A related process occurs in ordinary soil, not just in bedrock. Soil piping happens when water flowing underground erodes a channel through the soil itself, washing fine particles away and leaving behind a hollow tube. These pipes are invisible at the surface for a long time because the roof holds. Eventually, though, the roof collapses, and what looks like a sudden hole or trench appears.13Earth-Science Reviews. Subsurface erosion by soil piping: significance and research needs Soil piping is common on hillslopes, in agricultural fields with poor drainage, and in areas where groundwater levels fluctuate. The process has been modeled mathematically, and researchers can now predict the likely size and location of piping voids based on soil properties and water flow patterns.14Acta Geotechnica. A coupled hydro-mechanical model for subsurface erosion with analyses of soil piping and void formation
Tree roots also create holes, though on a slower schedule. When a large tree topples, its root plate pulls up a mass of soil, leaving a pit where the roots once sat and a mound of soil where the root ball landed. This pit-and-mound topography is a dominant feature of mature forest floors. Even living trees contribute: as roots grow and decay within the soil, they leave behind channels and cavities that gradually fill in or persist as pathways for water and air.
How Burrowing Animals Reshape Ecosystems
All of these holes are more than just cosmetic features. Burrowing animals function as ecosystem engineers, and their digging changes the physical and chemical properties of the soil in ways that ripple through the rest of the environment. The most direct effect is on water movement. Burrow networks create macropores, large channels that allow rainwater to penetrate the soil surface much more quickly than it otherwise would. A review of studies on burrowing animals and soil hydrology found that burrow characteristics like length, connectivity, and total volume all influence infiltration rates, surface runoff, and overall soil hydraulic conductivity.15Earth-Science Reviews. The inclusion of burrowing animals in soil hydro-physical equations and models: A review
In Australia, researchers have argued that soil disturbance by native digging animals is critical to maintaining healthy landscapes. The foraging pits left by animals like bandicoots and bilbies trap leaf litter, seeds, and water, creating micro-environments with more decomposition, lower soil density, and different chemical profiles compared to undisturbed ground nearby.16Ecological Management & Restoration. Soil‐disturbance by native animals plays a critical role in maintaining healthy Australian landscapes Where these digging animals have declined due to habitat loss or introduced predators, the soil surface becomes harder and more compacted, water runs off instead of soaking in, and seedling establishment drops. The holes themselves are a kind of infrastructure for the ecosystem.
Gopher tortoise burrows in the southeastern United States illustrate a different facet of this engineering role. Tortoise burrow density in longleaf pine savannas has a measurable positive effect on the diversity and evenness of the vertebrate community, functioning as a keystone influence.17Biodiversity and Conservation. Functional relationships reveal keystone effects of the gopher tortoise on vertebrate diversity in a longleaf pine savanna More burrows mean more places for snakes, frogs, mice, and insects to take shelter from fire, heat, and predators. The tortoise’s digging habit supports an entire web of species that depend on structures they cannot build themselves.
How to Identify a Mystery Hole
If you have found a hole in your yard, garden, or a hiking trail and want to figure out what made it, a few clues narrow the field quickly.
- Diameter under 1 cm: Likely an insect. Ground-nesting bees, wasps, and beetle larvae leave small, clean holes, often in clusters in bare or sandy soil.
- Diameter 1 to 5 cm: Could be a large insect (cicada nymph) or a small mammal (vole, chipmunk, mouse). Cicada holes appear suddenly in spring and come with small mud turrets. Mammal holes tend to be surrounded by scattered soil and may show claw marks.
- Diameter 5 to 15 cm: Medium-sized mammals like rats, ground squirrels, or moles. Mole holes are typically associated with raised ridges of soil (their shallow tunnels).
- Diameter 15 cm or larger: Larger mammals such as rabbits, foxes, badgers, groundhogs, or wombats. Badger setts often have multiple entrance holes with well-worn paths between them. Groundhog burrows typically have a large mound of excavated dirt beside the main entrance and a concealed bolt-hole nearby.
- Irregular, slumping depressions: Likely geological. Soil piping collapses look less like a dug hole and more like the ground has sagged inward, often with cracked edges.
The soil around the hole also tells a story. A neat pile of granular earth suggests a digging animal that brings soil to the surface one load at a time. A fan-shaped spread of pale subsoil indicates something excavating deep, like a badger. No visible soil pile at all, combined with a clean round hole, points toward a burrowing insect. And a hole with no surrounding disturbance that appears after heavy rain is almost certainly water erosion, whether from piping, dissolution, or washout of a pre-existing void.
When Holes Become a Problem for Infrastructure
For the most part, holes in the ground are ecologically beneficial or at least neutral. But there are settings where burrowing creates real hazards. Earthen levees and embankments are particularly vulnerable. Animal burrows that penetrate a levee body create pathways for water to seep through during a flood. A study of 21 levees along a river in northwestern Italy found that when animal burrows were present, the critical conditions for internal erosion were reached during shorter flood durations, and the probability of water seeping through the levee increased, potentially forming longer erosion tunnels inside the structure.18Journal of Flood Risk Management. Impact of animal burrows on earthen levee body vulnerability to seepage Burrowing animals like nutria, muskrats, foxes, and badgers are the usual suspects in these cases. Levee managers spend significant resources on monitoring for and repairing animal burrow damage.
Soil piping causes problems for roads, foundations, and agricultural land as well. A field that develops piping voids can suddenly lose soil support, leading to collapse of the surface underfoot or under machinery. In hillside settings, piping can trigger or accelerate landslides. Because the pipes are invisible until the roof fails, the hazard is difficult to predict without specialized surveys.
How Underground Bodies Match Underground Tunnels
The physical design of a burrowing animal is tightly matched to the soil it lives in and the tunnels it builds. Subterranean mammals range in weight from less than 50 grams (like tiny mole-rats) up to a couple of kilograms, and they dig tunnels with diameters ranging from about 3 to 15 centimeters. The relationship follows predictable scaling: heavier animals dig wider tunnels, but tunnel size is not just about the animal fitting inside. The tunnels also need to stay structurally stable given the mechanical properties of the local soil. Researchers have developed mathematical models linking the maximum tunnel dimensions that a given soil type can support to the body mass of the animal digging it.19Journal of the Royal Society Interface. Burrowing below ground: interaction between soil mechanics and evolution of subterranean mammals In loose, sandy soil, tunnels collapse more easily, so animals there tend to build narrower passages. In dense clay, the walls hold up better and tunnels can be wider relative to the animal’s size.
Frogs and crabs take entirely different mechanical approaches. The headfirst burrowing frog described earlier uses its forelimbs to scrape soil away, a strategy that works well in soft, damp ground but would fail in compacted earth. Fiddler crabs use their walking legs and smaller claw to scrape and carry sediment out of their burrows pellet by pellet, which is why their holes tend to appear in soft mud and sand rather than hard substrate. Ants, meanwhile, excavate one grain at a time with their mandibles and carry each grain to the surface. This painstaking method is why ant colonies take weeks to build a deep nest, but it also means ants can dig in surprisingly hard soil where larger animals cannot.
Even earthworms vary their digging technique based on soil conditions. In loose soil, they push particles aside by expanding their body segments. In harder ground, they ingest the soil and pass it through their gut, depositing it at the surface as the familiar castings. The result in both cases is a network of tunnels, but the method differs completely depending on what the worm is working with.
Ancient Burrows in the Fossil Record
Burrowing is not a modern invention. The fossil record contains trace fossils of burrows extending back hundreds of millions of years, and these ancient tunnels are one of the main ways paleontologists study the behavior of organisms that left no skeletal remains. In one striking example, researchers in southwestern Australia found burrows in rocks that are 1.7 billion years old. The rocks themselves date to the Paleoproterozoic era, but the burrowing happened much more recently, during the Eocene epoch, roughly 40 to 50 million years ago. At that time, a marine transgression flooded a weathered land surface, and the ancient metaquartzites had softened into friable sand, allowing animals to burrow into them. The resulting trace fossils are indistinguishable from well-known burrow types produced by crustaceans and other marine invertebrates.20Proceedings of the National Academy of Sciences. Eocene animal trace fossils in 1.7-billion-year-old metaquartzites The finding is a reminder that the simple act of digging a hole in the ground is one of the oldest and most widespread animal behaviors on Earth, and it has been shaping landscapes and ecosystems for far longer than any of the species doing it today have existed.