Subsurface means beneath the surface of something, the hidden layer that lies below what you can see from above. When geologists drill into the subsurface of the Earth, they're exploring the rocks, minerals, and layers that exist underground, far below the soil you walk on.

Scientists study subsurface oceans on moons like Europa, where liquid water might exist beneath thick ice. Engineers examine the subsurface before building large structures, checking what kinds of rock and soil lie underground to make sure a foundation will be stable.

You can use subsurface as an adjective too: subsurface mining happens deep underground, and subsurface temperatures get warmer the deeper you go into the Earth. The subsurface world is vast and mostly unexplored, containing everything from underground rivers and caves to ancient fossils and valuable minerals, all hidden just below the ground we see every day.