If the oceans vanished, you could walk between continents—but you'd need a spacesuit. Down in the empty basins, atmospheric pressure doubles and the heavy air heats to a blistering 122°F.
The edges of the continents are ringed by shallow continental shelves, which appear as the bright green and light blue margins on a bathymetric map. In places like the Bering Strait between North America and Asia, the seabed is only about 50 meters below the current surface. If the water receded even slightly, massive land bridges would appear, creating flat highways.
However, once a traveler reaches the edge of the shelf, they face the continental slope—a descent dropping 10,000 to 15,000 feet to the true ocean floor. The abyssal plain makes up more than half the planet's surface. Crossing the Atlantic Ocean means trekking across thousands of miles of deep, viscous silt while navigating the Mid-Atlantic Ridge, an active volcanic mountain range spanning the length of the basin.
The terrain itself poses a massive challenge, but the true threat lies in atmospheric physics. Without oceans, Earth's atmosphere would sink, settling deeply into the newly empty basins. Following the barometric formula, air pressure increases with depth, making the air incredibly dense by the time a hiker reaches the average ocean depth of 3,700 meters (12,000 feet).
This sinking air undergoes adiabatic compression, warming by nearly 10°C for every kilometer of descent to create that blistering baseline environment. The deeper the trench, the more lethal the conditions become. Any traveler attempting a shortcut through the Mariana Trench would face pressures high enough to cause nitrogen narcosis and temperatures exceeding 120°C (250°F)—breathing air hot enough to bake them alive.