Down to a Sunless Sea — David Poyer

Depth: 105... Wriggling like an eel, he forced himself up into the narrow gap he'd created...Rocks scraped and clattered away. A hint of doubt--Should I be doing this?--was answered by the calm comfort that pervaded his mind now. Nothing was going to happen.... Everything was going to be all right...

Welcome to the most dangerous sport on Earth: where cave divers step into murky Florida ponds and end up hundreds of feet beneath the ground, slinking through Swiss-cheese-like rock formations, past strange underground creatures, heading down tunnels that may open into caverns, or lead nowhere at all.

Open-water diver and ex-Navy SEAL Tiller Galloway has come to this watery underworld to find out why an old friend died young--and take one last shot at being a father, a lover, and a friend. But with a woman who is opening up her heart, and a conspiracy closing in around him, Galloway must navigate between lies told aboveground and truths hidden in the depths--where a violent battle is about to explode...