I turned 45 in March and did the thing. Since I was a kid watching underwater documentaries on Sunday nights I'd wanted to scuba dive, and for about thirty years I found reasons not to: the money, the kids, the knees, the fact that I live 900 miles from an ocean. This spring my wife Denise said "you're going to be 45 whether you do it or not," which is the most persuasive thing anyone has ever said to me, so I walked into the dive shop in the strip mall by the highway and signed up. This page is the diary I kept. If you're in your forties and sitting on the fence: get off the fence. The water's fine. Well, the quarry's not fine. But the reef is.

I am not an instructor and this is not instruction. It's what one middle-aged guy went through, as accurately as I can remember it. Your shop and your course will have their own way, and you should listen to them and not me.

There are a couple of big training organizations, and the shops teach one or another's course, but the shape of the basic "open water diver" course is the same everywhere:

1. Classroom (or book and video at home) with quizzes and a final exam. The physics of pressure, how your ears and lungs behave underwater, the gear, the dive tables, what can go wrong and how not to let it.
2. Confined water: the pool. Several sessions where you learn every skill in water you can stand up in, then repeat them in the deep end.
3. Open water: four dives in a real lake, quarry or ocean, with an instructor, where you demonstrate the same skills for real, at depth, and do some actual diving.

Pass all three and you get a card that says you're trained to dive with a buddy without an instructor, in conditions similar to your training, to a recommended depth of about 60 feet. That's the whole deal. The card doesn't expire, but if you go a long time without diving, any decent shop will want you to do a refresher before they'll take you out, and they're right.

My course ran from late April to late June: three classroom evenings, five pool sessions, and a weekend at the quarry. It could be done faster; I had a job.

Seven of us: two college kids, a couple in their thirties planning a honeymoon, a woman about my age whose son had gotten certified and shamed her into it, a guy who never spoke, and me. Tuesday nights, in the back of the shop, with a TV on a cart and a whiteboard.

The stuff that stuck with me:

• Pressure. Every 33 feet of seawater adds another atmosphere of pressure. At 33 feet the air in your lungs, your ears, your mask, everything, is squeezed to half its volume. That's why your ears hurt on the way down and why...
• ...you never, ever hold your breath while you're breathing compressed air. It's rule one. It's on the first page and the last page. The air you breathe at depth expands as you come up, and if you hold it in, it has nowhere to go. Breathe normally, all the time, especially going up. I have this written on the inside of my logbook cover.
• The tables. You spend a whole night learning to read a laminated card that tells you how long you can stay at a given depth before the nitrogen in your body becomes a problem, and how long to wait between dives. I did badly on the tables quiz the first time. I am an accountant. I was mortified. Then I got it.
• Slow ascents and the safety stop. Come up slowly (the course said never faster than 60 feet a minute, and slower is better; watch your smallest bubbles and don't beat them to the surface), and on the way up stop at 15 feet for three minutes. Every dive. Even the shallow ones. It's cheap insurance.

Final exam: 50 questions. I got 47. The guy who never spoke got 50.

The high school pool, Sunday mornings before it opened, with the shop's rental gear laid out on the deck like an operating room. The first time I put a regulator in my mouth and put my face in the water and breathed, I was 45 years old and I giggled. Out loud. Bubbles everywhere.

Every session was skills, over and over, in the shallow end, then in the deep end:

• Mask flooding and clearing. Let water into your mask on purpose, then tilt your head back, press the top of the frame, and blow out your nose to push the water out. Everybody hates this one. Water up your nose, chlorine in your eyes. By the third session I could do it with my eyes open and not think about it. That was the point.
• Regulator recovery. Take the regulator out of your mouth, let it go, find it again by sweeping your arm back along the tank, put it back in, blow out to clear it, breathe. Then do it again with your eyes closed.
• Alternate air source. Your buddy's spare regulator (the "octopus"). Practice signaling "out of air," taking it, and swimming together to the surface. Practice being the one who gives it, too.
• Buoyancy. The fin pivot: lie flat on the bottom and add tiny puffs of air to your vest until your breathing alone rocks you up and down on your fin tips. Then the hover: cross your legs, sit in the water, touch nothing. The first time I hovered for a full minute I felt like I'd learned to fly. More on this below.
• Controlled emergency swimming ascent. Swim slowly up from the deep end while making a continuous "aaaaah" sound so you're never holding your breath. For if you ever run out of air with no buddy in reach. You practice it so you never have to think.
• Weight belt and gear off and on at the surface and underwater. Cramp removal. Tired diver tow. Snorkel to regulator and back. Over and over.

Five sessions, about three hours each. By the last one, the instructor was throwing problems at us without warning (knocking a mask sideways, turning off a valve for a second) and we were fixing them without panicking. That, it turns out, is what the pool is for.

If you learn to dive in the middle of the country, you do your open water dives in a flooded quarry, and there's one about two hours from here that a dive shop runs on weekends: a parking lot, a picnic shelter, a dock, and a hole full of green water with a sunken school bus in it for something to look at.

Water temperature: 68 degrees at the surface. 52 at 30 feet. There's a line about 20 feet down where the warm water sits on top of the cold, called a thermocline, and you can actually see it shimmer, and when your head goes through it it's like opening a refrigerator with your face. Everyone wears a 7mm wetsuit with a hood and gloves, and everybody is still cold. Visibility was maybe 10 to 15 feet. You could see your buddy and the line and not much else.

Dives 1 and 2, Saturday. Down the line to a platform at 20 feet. All the pool skills again, kneeling on the platform, in the dark, in the cold: mask clearing (with 52-degree water; I have never been so awake), regulator recovery, alternate air. Then a short swim around the platform following a compass heading and back. Dive 2, the same plus the emergency ascent from the platform. Max depth 22 feet. I was fine. I was shaking afterward, but I was fine, and it was mostly the cold.

Dives 3 and 4, Sunday. Deeper: down the line to the school bus at about 40 feet, a real dive with a real tour, then a navigation exercise (swim out on a heading, turn around, come back to the line; I came back eight feet from the line, which I was told was good), then the hover, then a proper slow ascent with a safety stop at 15 feet for three minutes, hanging on the line in the green watching my bubbles. Max depth 41 feet. Total bottom time for the weekend: about an hour and ten minutes.

The instructor shook my hand on the dock and signed my book. I sat in the truck with the heater on for twenty minutes and called Denise from the pay phone at the gas station and I think I was a little hard to understand.

Honest opinion: the quarry is not fun. It's cold and dark and there's a bus. But you learn to dive in a quarry the way you learn to drive in a parking lot: if you can do it there, you can do it anywhere, and when you finally get to the ocean it feels like a reward.

1. Equalize early and often. As you go down, the pressure squeezes the air in your middle ears and it hurts. You fix it by pinching your nose and blowing gently against it (or swallowing, or wiggling your jaw; different things work for different people). The trick everybody learns the hard way: do it before it hurts. Every couple of feet on the way down. Once it hurts, go UP a few feet, equalize, then try again. Never force it and never push through pain. I had a stuffy head one pool session and couldn't clear my left ear and the instructor made me sit that one out. He was right. If you have a cold, you don't dive. Period.

2. Buoyancy is the whole sport. You wear a vest (the BC, or BCD, buoyancy compensator) that you add air to or let air out of, and a weight belt, and the goal is to be neutral: not sinking, not floating, just hanging there. Get your weighting right first: at the surface with an empty BC and a normal breath you should float at about eye level. Then underwater, add air in tiny bursts and wait; it takes a second to feel it. And here's the thing nobody tells you: once you're close to neutral, you steer with your lungs. Breathe in a little deeper, you rise. Breathe out, you sink. New divers (me) flail their arms and kick up the bottom. Good divers barely move. On my ninth dive I finally felt it for a few minutes, that feeling of just hanging in the water, and it's the reason people do this.

3. The buddy system is not optional. You dive in pairs, always. Before you go in, you check each other's gear head to toe: air on, straps, weights, releases, does the regulator breathe. Underwater you stay close enough to touch in a couple of kicks, you check on each other constantly (the "OK" sign, thumb and finger in a circle), and you both know the plan. Your buddy is your backup air, your backup brain, and your witness. Denise is not certified, so my buddy in the course was the woman whose son shamed her into it, and we were a good pair: neither of us was in a hurry and both of us said so.

We'd had a family trip to the Keys on the calendar for a year, and the whole certification thing was secretly so that I could dive on it. Denise knew. Key Largo, the second week of August. Hot enough to melt the dashboard. I booked two mornings with a dive shop on the water, two tanks each, on a boat with a dozen other divers, most of whom had hundreds of dives and were very kind to the new guy with the shiny logbook.

Dive 5. Tuesday, August 10. A shallow reef a few miles offshore, 25 to 30 feet. Here is what I wrote in the log that night, and I'm leaving it exactly as I wrote it:

Water 84 degrees. No wetsuit, just a T-shirt. Viz 60 feet plus. Went down the anchor line and the bottom just appeared, all of it, at once. Colors. Yellow and purple fans waving. A whole school of little blue and yellow striped fish that let me swim right into the middle of them. A parrotfish the size of a house cat crunching on the coral, and you could HEAR it. Brain coral like a lumpy boulder. A barracuda hanging perfectly still off to the side just watching, and I did not care. A little ray on the sand that flew away. 42 minutes. Came up with 900 psi. Denise was on the boat and said I came out of the water talking and didn't stop for an hour. Correct.

Dives 6, 7 and 8. Three more reefs over the two mornings, all in the 25-to-40-foot range. A nurse shark asleep under a ledge, five feet long, which the divemaster pointed at and I nodded at and my heart went about 140. Sea turtle on dive 7, just cruising, not in a hurry, like it had somewhere to be but not right now. A lobster backing into a hole waving its antennas at me. A moray eel's head, poking out, opening and closing its mouth (that's how they breathe, the divemaster said afterward; it's not a threat). Lots of "OK" signs. My buoyancy was terrible on dive 6 and pretty good by dive 8. On dive 8 I hung upside down for a minute looking under a ledge and didn't touch anything and that's my proudest moment as a diver so far.

Dive 9. Thursday, August 12. A shallow, sandy spot with a small wreck on it in about 30 feet, the easy last dive of the trip. And this is where the buoyancy finally clicked; see above. I spent half the dive just hovering over a patch of sand watching a garden of little eels stick up and duck down. Denise snorkeled above me and waved.

The quarry made it possible. The reef made it worth it. Nine dives total. My logbook has sand in it.

Denise asked, so I kept track. This is what I paid, at my shop, this year. Your shop will be different. Prices in round numbers.

Is that a lot? Yes. Is it less than the boat Denise wouldn't let me buy in 1994? Also yes. Spread over the rest of my life, it's cheap. You don't have to buy the BC and regulator right away; plenty of people rent for years. I bought because I knew I'd keep going, and because I wanted gear I knew.

What's next: the shop runs a pool night once a month for certified divers to keep the skills sharp, and I'm going. There's an "advanced" course (deeper, navigation, night diving) I'm looking at for the spring. And Denise has said the word "Cozumel" twice now without me bringing it up, so I'm not bringing it up. The dive computer decision is still under construction.