Narrator: Why does Florida have so many springs? Florida sits on a layer of porous limestone which acts as an underground reservoir, or aquifer. This aquifer can absorb huge amounts of water.

Where the pressure is great enough, springs bubble up, spewing out billions of gallons of fresh water every day. Other springs barely ripple the sand.

This is water as perfect as it comes - pure enough for locals to drive a hole in the ground and drink.

The source ... is rain.

The rain embarks on a long journey which will take it underground and eventually turn it into spring water. Nearly all rivers in Florida run underground.

When it rains, the water absorbs acids from plants and soil.

These acids dissolve the limestone bedrock, opening up new cavities, forming tunnels below.

Underground, the water releases its minerals, drop by drop, forming stalactites.

Entering this underworld, one is transported to a wholly different place. It is not teeming with life. It's as dark and still as the moon.

These tunnels are among the last explored places on earth. Divers daring and able enough are lured in search of the unknown. Their risks have paid off. These tunnels and caves have offered up some amazing discoveries.

Like this blind cave crayfish. Scientists have determined that they can live here for more than a hundred years - twenty times longer than their surface cousins. No one can explain why.

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