Blue Velvet-- It's a "Grim Reaper" sign. Warns open water divers of the dangers of cave diving, they are placed by the National Association for Cave Diving, the National Speleological Society: Cave Diving Section, and PADI at many cave diving locations to try and prevent open water divers from entering the cave system-- they are often easy to get in, but a small mistake can kill you quickly if you are not trained and do not possess the required safety equipment. Below is a slightly better picture of the one put in the other entrance to the cave that the shot of me is from. (NOT a picture of me, though I did spend 20 minutes on a deco stop just to the left of the sign pictured, this afternoon)
The picture of me was taken in the Gallery, the front portion of the Little Devil's cave system located at Ginnie Springs in High Springs, Florida. I am approximately 65 feet deep. The guideline for the cave starts right about there, and there is roughly 6.5 miles of underground passage. It's my "home cave," located about 30 minutes from me, so I dive there often. Just finished a two and a half hour dive there this afternoon, actually, though the picture is one a friend took two months ago or so which I found today, one of the few pictures of me diving.