Below the surface of something, especially below the surface of the ground or water.
The word subsurface makes people think of things that are hidden or not easy to see.
Examples:
The subsurface water is very cold.
Subsurface mining is dangerous work.
The subsurface of the ocean is full of interesting creatures.
I think there are subsurface tunnels under the city.
The subsurface world is a mysterious place.
Subsurface pipes carry water to the city.