It didn't take long for Dan Carlock's dive to turn potentially fatal – but it took him a while to realize it. The 45-year-old Los Angeles man boarded the scuba boat Sundiver in Newport Beach, California, on the morning of Sunday, April 25, 2004, bound for an oil rig nine miles offshore. He planned to explore the platform's 700-foot legs with his dive buddies, but as he descended he couldn't equalize the pressure in his ears. Carlock surfaced to find himself alone, surrounded by fog, with no boat in sight. He assumed the Sundiver would come looking for him. But the dive master had botched the roll call and, thinking all his clients were onboard, moved on to another site without any inkling that Carlock was missing. As the hours passed, Carlock drifted, and lost hope. He began snapping photos of himself to document what he was sure was his imminent death.

The possibility of being left in the open sea ranks among a diver's worst nightmares. The fear is bound to spread this summer, with the release of 'Open Water,' a low-budget indie film that's being called the 'Blair Witch Project' of the air-tank set (see "What If You're Not Alone?" below). As one character says to another after they've both been left adrift, "It happens more often than you'd think."

In fact, situations like Carlock's are far from common. "In the last five years I've heard of only four or five cases," says Captain Michael Karr, chief of the U.S. Coast Guard's Office of Investigations & Analysis.

The real-life drama that inspired 'Open Water' took place off the coast of Queensland, Australia, in January 1998, when Louisiana couple Thomas and Eileen Lonergan disappeared on the Great Barrier Reef after their boat's crew bungled a roll call. The alarm wasn't raised until two days later, after the captain discovered the Lonergans' belongings in a dive locker. Despite a massive search effort, they were never found. Ten days later some of their gear washed up 43 miles down the coast, and six months later a dive tablet turned up bearing a chilling message: please help us or we will die. January 26, 8:00 a.m.

What can you do to make sure this doesn't happen to you? In the wake of the Carlock incident, the coast guard called upon the Professional Association of Diving Instructors (PADI) to institute procedures for dive masters, such as written checklists or numbered wristbands, to make sure all divers are aboard at the end of each dive. Meanwhile, the Divers Alert Network is promoting its Diver ID System, in which a numbered tag clipped to each diver's buoyancy device must be collected before the boat departs a site. But ultimately, says the coast guard's Karr, "You've got to take responsibility for yourself. Go to the master and ensure that he has a process for not leaving anybody behind." PADI also recommends carrying whistles, flares, or mirrors, as well as shark-repelling devices.

That's not enough for Carlock, who was finally rescued after four hours, when he was spotted by a tall ship carrying a group of Boy Scouts. "Dan's concern is to make the industry safer, and he's exploring options for doing that," says Scott Koepke, Carlock's lawyer. "He wants to make sure this doesn't happen to anyone else."

WHAT IF YOU'RE NOT ALONE:

A new film tells the tale of two stranded divers and their unwelcome companions.

The divers in the film 'Open Water' are stranded at sea much like Dan Carlock – but the waters are swarming with bloodthirsty sharks. Director Chris Kentis talks about the shoot, the sharks, and his own near-miss.

Have you ever been left behind during a boat dive?
Once I was diving off St. Vincent, and when I surfaced I couldn't see the boat. It never occurred to me that the boat wouldn't show up – we always just think we're going to be okay. And 10 minutes later it did.

Was working with sharks terrifying?
We used a shark-wrangling outfit in Nassau [Bahamas] run by real pros. We weren't going to fool around. They had the actors wear antishark chain mail under their wet suits.

Any close encounters while filming?
The sharks were constantly bumping into me. I didn't wear the armor, because it was heavy and I was operating the camera. Twice I had sharks lunge and bite at my camera, and I had to yank it from them.