not probably as close to the surface as this, but you definitely have cavities of this size all over the state of florida. >> reporter: fossils found in this sinkhole show it's been around since the ice age. but no different than the sinkholes we see opening up today. these are just a few of his pictures. but one thing they all have in common is water. >> rain water is going to turn into ground water and that's what's naturally acidic. that's the device that dissolves the limestone and will help create these cavities. >> reporter: what is unusual about this sinkhole, it's easy to get inside. called the devil's den, it's open to tourists for viewing and diving. and dive instructor prince johnson takes me under for a look. i find that this seemingly placid pool of water is anything but. >> the water has gone down considerably because of the aquifer, but it's also risen when we've had hurricanes and tropical storms, it has risen