The aquifer stretches under the whole of Florida and parts of four other southern states.

In fact, there's more water held in underground aquifers like this than in all the lakes and rivers in the world.

Up above, we've successfully locked onto the divers' position 100 feet below.

Hello, Richard. Can you hear me?

Tom, how are you? How you doing? What do you see already?

We're walking with you now, Tom.

We think we've got you. We're marking your path.

Brian, all this that we're doing, mapping it on the surface, linking it to the underground.

Is it useful?

The water in this aquifer is the source of most people's drinking water in Florida, so by mapping we can help to prevent development over the top of the caves.