"The surface is not your friend." Tim Gallagher's few words say it all when your body is saturated with nitrogen and you are running low on air. This is not the time to make mistakes, even mistakes as simple as tangling your fin strap in the line you are using to find your way back to the safety of Aquarius. He reminds us again and again. Later, when I inhale a couple shots of salt water during a surface rescue breathing exercise in six-foot seas, it occurs to me that having a disabled diver is "not your friend" either!

Tim is one of the trainers leading four scientists and two technicians through drills for an upcoming 10-day mission to Aquarius, the only underwater laboratory in the world. He is an accomplished cave diver, which comes in handy when training people for work in Aquarius. He reminds us that what we will be doing is the same as diving in an "overhead environment," a fancy term used by cave divers and commercial divers working in pipes, where up only means hitting your head. We will be living in the habitat at 47 feet deep, and doing excursions on scuba to depths of 115 feet for up to nine hours a day. The only way to surface is to undergo 17 hours of decompression. Until then, he reminds us, "The surface is not an option."

This will be my third saturation mission in Aquarius. I set up and sampled 22 monitoring stations on Conch Reef, off Key Largo, Florida, in 1994, and revisited them again in 1998. The idea behind doing this work from the habitat was to begin monitoring coral communities at depths beyond those commonly studied by diving scientists. Shallow water communities may be subject to more natural disturbance than those in deeper water. This complicates one's interpretation of observed changes. Were the changes caused by storms or do they indicate some sort of real environmental degradation? Changes we see at 65, 95, or 110 feet, where we set up our stations, are more likely to reflect changes in the quality of the environment.

I'm training with Dr. Steven Miller, Dione Swanson, and Allison White, the other scientists on this coral reef monitoring mission. Our training for the mission consists of lots of different drills and exercises. Tim and Mark Hulsbeck run us through drills to find lost buddies, drills to locate lost lines that could lead you home, compass drills, drills to rescue disabled divers, drills to isolate and stop air leaks, buddy breathing and air sharing drills. We will do this for four days prior to descending to Aquarius for an additional 10 days. Each day gets a bit more complicated. We're told that our contact lenses had better be "sacrificial" on the third day, when drills will be done without masks.

"Safety first, then science." I wouldn't call it their motto, but the trainers sure say it a lot around here. My reasons for being here revolve around science, but there is not much talk about it during our training. I listen anyway. I listen because I know they are right. I trust these people completely. I have been through this two times before, and I know that if I follow their advice, I will never have to use the drills we are practicing so intensely this week. These are perhaps the best, and certainly the most conscientious, trainers I have ever met.

Saturation diving is serious business. "The surface is not your friend" is more than a phrase that catches your attention. Within a few minutes of an accidental surfacing, a saturated diver will experience joint pains, nausea, or a host of other debilitating symptoms of the bends. If you still have air, you can take a quick compass bearing to the large Life Support Buoy that floats above the habitat, and descend to the bottom to head home. If you are out of air, your best bet is that constant reminder of last resorts, a VHF radio inside a waterproof case on our hip, used only when you get to the surface to utter that dreaded phrase "mayday, mayday," while you pray for company. For a boat "mayday" means that you're going down. But to an aquanaut it means, "I'm on the surface and would give anything to get back down."

Tim was new to the Aquarius staff in 1998, when I did my last mission, but he did much of the training then too. I was impressed then, but I can see that he and the Aquarius staff have made a lot of improvements to the diving operation since.

I am reassured when the trainers explain exactly why every piece of equipment is configured the way it is, and how it works. Everything is purposeful. Two separate regulators cut the small chance of catastrophe even more. The primary regulator hose is 5 feet long and wraps under your arm then once around the back of our necks - a counter-intuitive arrangement, but it works and is comfortable. This configuration allows a diver to offer a long hose to a buddy who is low on air. A second regulator hangs inches below your mouth and is connected by a strap around your neck. You hardly notice it when diving, but knowing it's there could mean the difference between life and the alternative if "the monkeys come calling," as Tim says. A valve on the manifold that connects the two tanks allows the diver to isolate the tanks if a hose blows, saving half the precious air supply. Clips hold hoses close to the body to minimize drag, as well as fumbling by scientists with poor diving habits. Shears to cut trolling line that might otherwise drag you to the surface wait in your pocket. All equipment is of the highest quality, and is expertly maintained by the Aquarius staff.

After the day's drills, I remove the 100 pound tank-set and let Tim and Mark hoist it onto the boat. I strip off my wet suit. I feel the rain. My mind drifts to the Weather Channel, which was watching four tropical weather systems in the Atlantic when we left this morning. I wondered whether Hurricane Alberto or the three tropical waves had strengthened and turned our way. I wondered whether I would soon have a third chance to experience the thrill of Aquarius.