“Below sea level” means a point on land that sits lower than the average height of the ocean’s surface. If you could draw a perfectly flat line across the top of the ocean and extend it inland, any ground that falls beneath that line is below sea level. It sounds like it should be underwater, but geology, climate, and in some cases human engineering keep these areas dry.
How Sea Level Is Actually Measured
Sea level isn’t a single, fixed number. The ocean’s surface rises and falls with tides, currents, and weather, so scientists use an average. NOAA defines mean sea level as the average of hourly water level measurements taken over a 19-year period called the National Tidal Datum Epoch. That long timeframe smooths out tidal cycles, seasonal changes, and short-term weather effects to produce a reliable baseline.
There’s also an important distinction between local and global sea level. Local sea level is what a tide gauge measures at a specific coastal point. It reflects not just the ocean’s height but also whether the land itself is rising or sinking. In some places, the ground is slowly subsiding due to geological processes or groundwater extraction, which makes the local sea level appear to rise faster. In other places, land is still rebounding upward after being compressed by glaciers thousands of years ago. Global mean sea level averages all of these local measurements together to describe the ocean as a whole.
When a sign at a national park says “282 feet below sea level,” it’s telling you that the ground there sits 282 feet lower than this global average ocean surface.
The Lowest Places on Earth
The Dead Sea, bordering Jordan and Israel, holds the record as the lowest point on Earth’s surface at 418 meters (about 1,371 feet) below sea level. That number has actually been dropping further in recent decades as water diversion from the Jordan River causes the Dead Sea to shrink.
In North America, the lowest point is Badwater Basin in Death Valley, California, at 282 feet (86 meters) below sea level. If you visit, you can stand on the salt flats and look up at the cliffs of the Black Mountains to the east, where the National Park Service has posted a sign marking where sea level would be. The visual gap between where you’re standing and that sign is striking.
Other notable below-sea-level locations include the Turpan Depression in northwestern China (about 154 meters below sea level), Lake Assal in Djibouti (155 meters below), and the Qattara Depression in Egypt’s Western Desert (133 meters below).
How Land Ends Up Below Sea Level
Several geological processes create land that dips below the ocean’s average surface. Tectonic activity can pull the earth’s crust apart, forming rift valleys that sink over millions of years. The Dead Sea sits in exactly this kind of rift. In other cases, evaporation in enclosed basins removes water faster than rivers or rain can replace it, leaving a dry depression well below the surrounding terrain. Death Valley formed through a combination of tectonic stretching and faulting that dropped the valley floor while pushing the surrounding mountain ranges higher.
Human activity creates below-sea-level land too. The Netherlands is the most dramatic example. Much of the country consists of polders: areas that were embanked with dikes and then drained of water to create usable farmland. A network of drainage canals, sluice gates, and pumping stations keeps water levels in each polder carefully controlled. Without this infrastructure, large portions of the country would flood. The Dutch have been building and maintaining polders for centuries, effectively wresting land from the sea and keeping it dry through constant engineering.
What Changes Below Sea Level
Living or visiting below sea level comes with some measurable physical differences, though most are subtle. Air pressure increases as you go lower because there’s more atmosphere stacked above you. For every thousand feet of elevation change, atmospheric pressure shifts by roughly one inch of mercury. At Badwater Basin, the air pressure is slightly higher than at the coast, meaning the air is marginally denser and contains a bit more oxygen per breath. Most people won’t notice this, but it’s the exact opposite of what happens at high altitude, where thinner air makes breathing harder.
That extra air pressure also affects the boiling point of water. At sea level, water boils at 100°C (212°F). At higher elevations, it boils at lower temperatures because there’s less pressure holding the water molecules in liquid form. The reverse happens below sea level. At a depth of 1,000 meters below sea level, atmospheric pressure reaches about 114 kilopascals (compared to 101 at sea level), and water wouldn’t boil until roughly 103°C. At the Dead Sea shore, the effect is real enough that water boils at a slightly higher temperature than it does at the beach.
Temperatures also tend to run hotter in deep depressions. The surrounding terrain traps heat, and the denser air retains warmth more effectively. Death Valley regularly records some of the highest temperatures on Earth, partly because its below-sea-level basin acts like a natural oven.
Below Sea Level and Flood Risk
Any city built below sea level faces a permanent flood threat. New Orleans is the most prominent example in the United States. Substantial areas of the city, covering 318 to 426 square kilometers, already lie below the high-tide line. Between 386,000 and 448,000 people live in these zones, protected by an extensive system of floodwalls, levees, and drainage pumps. When Hurricane Katrina overwhelmed that system in 2005, roughly 80% of the city flooded.
The challenge for below-sea-level cities grows as global sea levels rise. Land that currently sits just above the waterline can slip below it, and areas already below sea level face deeper potential flooding. In places where the ground is also sinking, like parts of the Gulf Coast and the eastern seaboard, the gap between land height and water height widens from both directions. Maintaining the infrastructure that keeps these areas dry becomes more expensive and more critical over time.