The flattest place on Earth is determined by precise scientific measurement, moving beyond simple observation. Answering this requires distinguishing between a geometrically flat surface and one that is level relative to the planet’s curve. Geologists and surveyors use different metrics to evaluate extreme uniformity across varying scales, from square miles to entire ocean basins. The location considered the “flattest” depends entirely on the criteria used to measure its relief.
Defining “Flat”: The Scientific Metric
Flatness in a geological context is measured by the total elevation variance, or topographic relief, across a defined horizontal distance. Surveyors calculate this by determining the distance between two imaginary parallel planes that completely enclose the surface. The smaller this distance, the flatter the surface is deemed to be. Modern techniques rely on sensitive instruments, such as high-resolution GPS and laser altimeters, to detect minute elevation changes over vast expanses.
The true metric of geological flatness is a lack of gradient or slope, meaning the surface does not deviate significantly from the local horizontal level. While no natural terrain is mathematically perfect, the flattest locations exhibit variances measured in centimeters over many kilometers. This uniformity defines an exceptionally level surface, allowing scientists to use these features for global calibration tasks.
The Primary Contender: Salar de Uyuni
The location most frequently cited as the world’s flattest natural surface is the Salar de Uyuni, a salt flat situated high in the Bolivian Altiplano. This enormous expanse covers approximately 10,582 square kilometers, making it the largest salt flat globally. Its surface exhibits an average elevation variation of less than one meter over the entire area.
The Salar’s uniformity results from the evaporation of massive prehistoric lakes, such as Paleo Lake Tauca, which left behind a mineral crust of smoothness. This crust is primarily composed of halite, or table salt, and conceals a deep pool of brine rich in lithium. When a thin layer of water covers the resulting level surface, the Salar transforms into the world’s largest natural mirror.
The flatness of the Salar de Uyuni has a direct scientific application: it serves as a large-scale calibration target for Earth observation satellites. Space agencies use this site to fine-tune the altimeters on orbiting spacecraft, which measure changes in sea level and ice sheet thickness. The precise, known elevation provides an invaluable ground reference, which is more stable than the open ocean surface typically used.
Comparing Terrestrial and Oceanic Flatness
While Salar de Uyuni is the flattest terrain on land, the vast Abyssal Plains of the deep ocean floor hold the title for the flattest regions by scale. These underwater plains lie at depths of 3,000 to 6,000 meters and cover more than 50% of the Earth’s surface. They are characterized by a minute gradient, often less than a 1:1,000 slope, meaning the elevation changes by less than one meter per kilometer of horizontal distance.
The difference lies in the nature of the flatness: the salt flat achieves microscopic smoothness over a small area, while abyssal plains maintain an imperceptible gradient over thousands of square kilometers. For instance, the Sohm Plain in the North Atlantic spans roughly 900,000 square kilometers, dwarfing any terrestrial flat. A terrestrial salt pan is locally flatter, but the abyssal plain is flatter in terms of its immense, large-scale uniformity.
Geological Processes That Create Flatness
The formation of the world’s flattest places is governed by the equalizing force of gravity acting on fluid materials, such as water or fine sediment. For salt flats, the process begins when water collects in a closed basin where evaporation rates exceed precipitation. As the water disappears, dissolved minerals, predominantly salt, precipitate out and crystallize.
This crystallization occurs across the entire basin floor. Because water always seeks the lowest possible point, the resulting mineral layer is distributed perfectly level. Each new layer of salt builds upon the last, effectively self-leveling the surface into a uniform crust. Over thousands of years, this process creates the extreme planar characteristics observed in locations like the Bolivian Altiplano.
The flatness of abyssal plains is created through geological burial occurring over millions of years. New, rough oceanic crust forms at mid-ocean ridges, but as the crust moves away, any uneven topography is slowly covered. Fine-grained materials like clay, silt, and the microscopic remains of marine organisms, known as pelagic rain, continuously settle on the ocean floor.
These fine sediments are also delivered by powerful underwater landslides called turbidity currents, which fill in and smooth out pre-existing hills or depressions. This slow, continuous deposition of sediment creates a vast, gently sloping layer that can reach thicknesses of a kilometer or more, blanketing the underlying rugged basalt and forming the planet’s largest level surfaces.