Think of a ten-story building. In your mind, tip it upside down. Now picture how far the top of the building would reach under water. That's about 100 feet down. That's where I was headed at 10:00 on a hot Thursday morning in August. Diving to a shipwreck, 100 feet straight down.

I was three miles off shore in the warm ocean near Key Largo in Florida. I had already swum 20 feet down. There was a thin nylon line on my weight belt. It was snapped to a cable that dropped from the boat above. On the other end of the cable was a heavy anchor. Being hooked to this line made it easy for me to go straight down in the Gulf current.

Normally, I wouldn't use a guideline. I'd take my time and let the current just take me. But this was a work dive, not a fun dive. Normally, I would have a partner. Diving alone is not smart. But my Uncle Gord couldn't afford to hire another diver. So just this once, I was alone. Besides, all I had to do was go down, then up. Simple and safe.

At 30 feet deep, I tilted my head to look up at the surface. The shadow of the GypSea -- my uncle's dive boat -- was a long black shape. It looked like a fat cigar floating above me.

Far below me was another boat. This one was much larger than the GypSea. And it was in pretty bad shape. The shipwreck was called the Duane. It was an old Coast Guard ship. There was no cool story about it sinking in a storm or anything like that. The Duane had been cleaned up and then sunk on purpose. It was sunk to make a reef, a nice hiding place for fish and other undersea animals. It was also sunk so tourists could scuba dive and explore it.

Tourists were part of my job. In fact, I was diving down to the wreck so I could hide a toy treasure chest for them. It was my idea. Business had not been good for a while. More people might hire my uncle's boat for dives if we set up a treasure hunt for them.

The toy treasure chest I carried was not much bigger than a football. Inside it was a hundred-dollar bill sealed in a plastic bag to keep it dry. We were hiding it deep for experienced divers.

I kicked my fins and swam down another ten feet. Slowly.

I don't like to hurry when I scuba dive. I also check and double-check everything. All the time. There is a saying in this sport: There are old divers and there are bold divers, but there are no old, bold divers. In other words, mistakes can kill you.

I dropped another 10 feet. I was down to 50 feet. I stopped and hit a button to pump some air from my tank into special pockets built into my vest. I did this because, as you go deeper, the weight of the water makes it harder to swim. By adding air to my vest, I was able to make myself lighter.

As I swam, I turned my head and watched for sharks. Especially hammerheads. Around Key Largo, they can be as long as a car. But much more dangerous. Cars just need gasoline for fuel. Sharks need meat and blood. I didn't want to be a quick fill up for a shark.

I didn't see any sharks. I saw plenty of smaller fish. Although I knew they were very colorful, they looked bluish gray. Even clear water soaks up colors. After 50 feet, reds and oranges and yellows are gone. The blue colors go after 60 feet.

At 65 feet deep, I checked the dial on my air tank. Solo diving needed extra care. Above me, in the GypSea, a guy named Judd Warner was waiting for me. He had just been hired by my uncle. Judd expected me back in a half hour.

At 70 feet deep in the water, I swallowed hard and popped my ears, something I had been doing all the way down. I did this to keep my eardrums from exploding.

At 75 feet, the ship below me was as big as a football field. I was only a few minutes away from it. It seemed ghostly in the dim water.

I finally reached the shipwreck at 100 feet. I found a place to hide the treasure chest and began to rise again.

It happened fifteen minutes later on my way up. At 53 feet deep, something ripped my mouthpiece away from my face. And the water around me exploded.

I was still attached to the cable that was my guideline. For a couple of seconds, I bucked and danced at the end of my line. I felt like a rag doll shaken by a giant. Air bubbles kept exploding around my face mask. I grabbed my backup mouthpiece. It didn't work!

I tried to grab the main mouthpiece. At least it had air. The rubber tube was like a live snake. It twisted and turned in the water, trying to get away from me.

The air bubbles were coming from the mouthpiece. Air is squeezed into tanks under great pressure -- 3,000 pounds per square inch. A valve lets the air out slowly when you breathe. But the valve must have broken. The pressure was escaping, all at once. It was escaping through my mouthpiece. In a hurry. I was losing so much air and losing it so fast that the force of it was shaking my entire body.

I finally got my hands around the mouthpiece tube. I pulled the mouthpiece toward me. But there was no way I could put it back into my mouth. The air was shooting out too hard. Trying to breathe air from it would have been like trying to sip water from a fire hose.

But I needed air. Badly. And soon. I was 53 feet under water.