The idea of a city sinking beneath the waves is often relegated to myth, but this phenomenon is a historical and contemporary reality. “Underwater cities” fall into two categories: ancient archaeological sites swallowed by natural forces, and modern metropolises facing imminent submergence. Examining the ruins of the past and the risks to the present helps us understand the powerful geological and human-induced forces that drive communities beneath the water.
Cities Submerged by History
Ancient cities that now lie on the seabed often serve as preserved snapshots of catastrophic events. One prominent example is Port Royal, Jamaica, a 17th-century pirate haven largely destroyed in minutes. On June 7, 1692, a massive earthquake struck the city, causing the sandy land upon which it was built to undergo liquefaction. Two-thirds of the city slid into the sea almost instantly, preserving buildings and artifacts as a unique archaeological site.
The ancient Egyptian cities of Thonis-Heracleion and Canopus, now submerged in Abu Qir Bay near Alexandria, were claimed by a different set of forces. These cities disappeared around 1,200 years ago, a fate attributed to a combination of factors. Geological instability, including seismic activity, caused the land to subside slowly over time.
This gradual sinking was compounded by rising sea levels in the Mediterranean, a long-term process that ultimately overwhelmed the low-lying settlements. The submerged ruins, which include colossal statues and temples, offer a tangible record of how tectonic shifts and natural sea-level changes can erase prosperous urban centers from the map.
The Mechanisms of Submergence
The process by which dry land succumbs to water is driven by two compounding factors: land subsidence and eustatic sea-level rise. Subsidence is the sinking of the ground itself, which is often a localized and accelerated issue caused by human activity. When large volumes of groundwater are extracted from underlying aquifers, the resulting empty space causes the layers of soil and clay above to compact.
The sheer weight of dense urban development, with its heavy infrastructure, further compresses the water-saturated sediments, rapidly accelerating the sinking. In many coastal cities, the median rate of land subsidence, measured at about 16.2 millimeters per year, is significantly faster than the global rate of sea-level increase.
Eustatic sea-level rise is a global process referring to the increase in the total volume of ocean water. This rise is primarily caused by the thermal expansion of seawater as it warms and the melting of massive land-based ice sheets and glaciers. While the current global average rate of sea-level rise is approximately 3.7 millimeters per year, its effect is universal, increasing the baseline vulnerability of all coastal areas. When a city’s rapid subsidence combines with this steady global sea-level rise, the relative sea level—the height of the water compared to the land—increases dramatically, leading to more frequent and severe flooding.
Modern Metropolises Facing the Tide
Today, the most pressing examples of submergence are thriving population centers fighting the rising waters. Jakarta, Indonesia, presents one of the most alarming cases globally, with parts of the city sinking at rates up to 28 centimeters per year in the worst-affected areas. This extreme subsidence is overwhelmingly linked to the excessive and largely unregulated extraction of groundwater for residential and industrial use.
The crisis has become so acute that the Indonesian government has made the decision to relocate its capital to a new city on the island of Borneo. This planned move acknowledges the immense challenge of mitigating the combined threats of sinking land and chronic tidal flooding in the existing capital.
Another city wrestling with this dual threat is Venice, Italy, which sits on a foundation of soft sediments in a shallow lagoon. The city has experienced subsidence of roughly half a meter over the centuries, a problem now exacerbated by sea-level rise and increasingly frequent high tides, known locally as acqua alta. To combat this, Venice completed the ambitious MOSE project, a system of 78 massive, submerged floodgates across the lagoon inlets. These gates are raised when a dangerous high tide is forecast, forming a temporary barrier that successfully protects the city from water levels exceeding 130 centimeters. Colossal engineering projects and policy decisions, such as building seawalls or regulating water use, represent the global effort to protect modern populations from this ongoing risk.