The team arrives at sunset in Georgetown before heading the next morning to Silver Glen. But just because the houseboats are anchored, it doesn't mean the work is over: The film crew sets up lights so it can film the evening's dinner conversation between the scientists on board.

The steamboats that traveled this river in centuries past burned big knots of trees on their upper decks to dispel the pitch blackness of nights in the mosquito-infested Florida wilderness. The lighting of tiki torches on the Miss Jane is for ambience only.

What kills more people annually than gators in this part of the world? Lightning, of course. This spectacular display sends our crew running for cover even though it's more than 100 miles away from our houseboats anchored on the St. Johns; luckily, the storm stays well out over the ocean, east of Daytona Beach.

Bound for Silver Glen, the houseboats cross Lake George. It's a formidable 72-square-mile bulge in the middle of the St. Johns that can be mirror-calm one minute and then rougher than a white-capped ocean the next.

Producer Jill Heinerth passes after-work hours in the darkest bowels of the houseboat, logging tapes in her cramped living quarters-cum-production studio.

Biologist Tom Morris fuels his already encyclopedic knowledge of the natural world with a bit of research during a quiet moment before diving into Silver Glen. One of the foremost underwater cave divers in the world, Tom's claim to fame is a cave-dwelling crayfish named after him.

Wes plucks and strums his way to relaxation inside the houseboat before setting out for the first of several grueling dives into Silver Glen.

An idyllic wooded setting of palms and oaks surrounds the headspring at Silver Glen.

Silver Glen Spring, a poplar spot among locals and tourists, is often packed to the gills with recreational boaters, both human and canine.

Tom Morris prepares for one of several dives the team will make into Silver Glen to film the cave-diving sequence of the documentary.

Producer Jill Heinerth leaves behind the hulking houseboats, preferring the quiet of a kayak as she heads toward the spring.

Director Wes Skiles films from several different angles as Jill's kayak passes overhead.

Jill gives the universal dive sign indicating "OK." She's ready to begin the dive into Silver Springs. She, Wes and Tom intend to explore a massive watery cavern deep under the earth, but first the team must endure a long swim against a formidable flow through a narrow and twisting tunnel.

Jill follows Tom's lead as he prepares to enter the cave.

Tom enters the cave, a solitary and serene (if unforgiving) place that feels a million miles away from the civilized world just over his head on the surface of Silver Glen.

The initial passage, though it may appear foreboding to the uninitiated, isn't especially challenging for veteran cave explorers.

The tunnel that leads to the cavern grows progressively skinny and constricting, to the point that the divers describe it as "coffin-like."

A big man with a large camera in tow, Wes is undeterred by having to squeeze himself and his equipment through a tight crevice.

The team is rewarded for taking the risk and making the effort as a spectacular cavern opens out in front of them.

The patterned sedimentation deep below the surface of Silver Glen provides clues about Florida's geologic and hydrologic history.

Jill measures the salinity of the water that flows through the cavern. Described as "connate," this salty stuff is actually relic seawater that's been trapped since the ocean receded and the landform now known as Florida took shape.

Mullet swim in the undulating eelgrass grows around the Silver Glen headspring.

Pincer-heavy blue crabs are the fiercest-looking of the salt-water species that stand guard at the entrance to the cave at Silver Glen.

Atlantic Stingrays glide up and down the length of the St. Johns River, and are easily spotted in the clear waters of Silver Glen.

Striped bass are seasonal visitors to Silver Glen. When the main stem of the St. Johns gets too warm for their liking, they gravitate here to cooler spring water. The fish hang out here for months without eating until venturing back into the river.

Florida Department of Environmental Scientist Jim Maher measures the dissolved oxygen in Lake George. Major storm events (such as the hurricanes in 2004) can wreak havoc on oxygen levels, causing them to dip sharply and leaving resident fish populations virtually suffocating.