When someone discovers I scuba, they invariably ask, “How deep do you go?” It’s a fair question — many are not aware of deeper dive limitations with scuba in general or here in waters off Maine.

According to the website of the Professional Association of Diving Instructors, the limit for recreational scuba diving is 40 meters, or 130 feet. It is recommended beginners or less experienced divers not dive below 18 meters or 60 feet. It also depends on where you are diving, New England waters present different challenges than those of the Caribbean.

I was at a nephew’s wedding at Lake Tahoe and got a chance to dive. Way up in the mountains, it meant the dive was an altitude dive. We had to take the high elevation into account in our calculations as to what our acceptable dive times and no decompression limits would be. I ended up diving down to about 110 feet and visibility in that clear mountain lake was still incredible!

For my Advanced Open Water certification, we worked on specialty types of dives like Night Dive and Deep Dive. At a lake over near Waterville, we motored out to the middle and got briefed regarding skills for deep dives. Depth at that spot was 75 feet or so, about as deep as we could find.

I was given a padlock, one with the dial of numbers on it, and told the combination. My job was to go down to the bottom with the instructor and unlock the padlock. Sounded simple enough, but the deeper one goes in the water, the more nitrogen builds up in one’s system. This is why deep diving is a whole different kind of diving.

Scuba divers use air tanks, not oxygen tanks. The air in my scuba tank is just regular air made up of nitrogen (78.084%) and oxygen (20.946%), with the remaining percentage a mixture of carbon dioxide (0.033%), argon (0.934%), and traces of helium, neon, krypton, hydrogen, xenon, and carbon monoxide that together total about 0.003%. It is just like the air we breathe here at the surface.

The highest percentage in the tank is nitrogen, and as the density of nitrogen and its partial pressure increases with depth, it means the deeper you go, the more nitrogen molecules are being inhaled with each breath. Thus your body — including blood and soft tissues — absorbs those nitrogen molecules once you are underwater. And that absorption rate increases with depth.

With the breathing of air under pressure, the deeper you go, the more nitrogen enters your blood and builds up in your tissues. This is all well and good, but you need to let those built-up molecules dissipate before you reach the surface.

You do that by either not going very deep for very long, or by having a plan to make decompression stops at various depths as you ascend back to the surface. There, you can off-gas, i.e., allow the nitrogen to dissolve from your tissues.

That nitrogen build-up can cause problems if not allowed to dissipate. Without allowing time for them to dissipate, nitrogen bubbles might collect in your joints and, upon surfacing, can literally twist you into painful shapes. This is the diver malady called “the bends.”

Known as Decompression Illness (DCI), this ailment is made up of both Decompression Sickness (DCS) and Arterial Gas Embolism (AGE). DCS results from bubbles in body tissues causing local damage. AGE occurs when bubbles enter your circulation, travel through arteries and cause tissue damage by blocking blood flow at the small vessel level.

The old dive tables used in the early days of scuba to calculate time at depth. Photo by Charles H. Lagerbom

Back in 1979 when I first got certified in high school, we used dive tables to figure out our dive plan. They were rigid guidelines, based on U.S. Navy divers, as to how long you could or should stay at a certain depth. This was all to avoid the bends.

The tables were also used to determine how long our surface interval time should be before we went back to certain depths on a second dive. This is because of the cumulative effect of nitrogen build-up in the bloodstream. It needs time to dissipate or decompress from the tissues.

Today we use dive computers to do all that calculating. The computers also recognize that a dive is not just to a certain depth for a certain time, that dives could be deep, then shallow, then deep again, never static but changing throughout. The old dive tables never allowed for that. The computer works that into your bottom time so you don’t have to.

For recreational divers, your computer will let you know if you are approaching your non-decompression dive limit (NDL). If you do, it will then calculate what depth and for how long you will need to remain at that depth while off-gassing or decompressing, allowing nitrogen to dissolve from your system before you can safely surface.

As recreational divers, we always try to “plan our dive and dive our plan.” And the plan is to never exceed our NDL. We do that by arranging our dives where the deeper phase is done first. This allows nitrogen to start dissipating while we are in shallower waters during the latter part of the dive. We also usually end a dive with a general safety stop for three minutes at 15 feet, just to be safe.

Depending on how far you exceeded your decompression limits, DCI can exhibit itself as anything from a headache, mild rash or muscle soreness to acute joint pain or even worse. It is something not to mess around with and is part of dive planning before you even get into the water.

This is why we generally stick to dives in the 10-to-50-foot range. As I tell people, after about 60 feet in Maine waters (whether lake, quarry or ocean), it gets pretty dark, cold and creepy.

On my Advanced Open Water check-out dive, I splashed from the boat and descended the anchor line with my instructor right behind me. As one goes deeper, your neoprene suit contracts from water pressure. The deeper you go, the more it compacts until at deeper depths its insulating quality is not all that great. So, going deeper also makes you less buoyant and decreases the warmth of your suit.

Going down the anchor line, I felt myself picking up pace, because the neoprene suit was compressing. While not quite hurtling to the bottom, I still had to slow myself with the line. I had been going fast enough that my ears and sinuses needed constant clearing. More on that later!

So, I landed on the bottom of this lake trying to pop my ears open while I gripped the anchor line in one hand and had a death-like grip on the padlock with my other hand. The water definitely felt colder; my 7mm-thick suit had compressed quite a bit.

In what little visibility we had at 75 feet, I saw that the anchor line just went right into the bottom. That was when I realized the anchor had gone down into muck and disappeared. Well, I soon joined it.

Settling into the muck on my knees, I must have sunk a couple or 3 feet into it. My computer afterward registered a depth of 78 feet, 3 of which I think were muck! Of course, what little visibility we enjoyed went to nothing as the bottom got stirred up. I put the padlock up to my mask so I could make out the dial and started working my way through the combination.

The author’s lobster gloves, three-fingered for warmth but not great for dexterity. From the collection of Charles H. Lagerbom

This was pretty difficult in the murky light and with thick neoprene lobster gloves. We had been issued lobster gloves, where you have a thumb and forefinger but the other three fingers are all together. They are supposed to keep hands warm but definitely take away dexterity.

As I tried to settle myself, I noticed something else. In fact, I found myself starting to giggle at the ridiculous situation in which I now found myself: wallowing in three feet of muck in near total darkness holding on to an anchor line, trying to control my breathing while shivering in the cold water, noticing my leaky mask, while attempting the combination on a padlock I was desperately trying not to drop, all with those darn lobster gloves on.

This is the other feature of nitrogen, that build-up at depth. It is called nitrogen narcosis. This depth intoxication or getting “narked” is like a drinking buzz or down-right drunkenness. It affects divers differently at different depths; for me it began about 70 feet or so.

Like alcohol, some handle it better than others. Enough effect, however and you start to do and think stupid things. I have heard of people wanting to sing out loud and read of one narked diver who tried to offer his regulator to a passing fish, thinking it desperately needed air. People have even died from this “rapture of the deep,” so it is nothing to mess with.

It took me three tries, but I got that hated padlock open. Another student diver landed beside me with a mini-marker board and started to work on some algebraic equations. I did not stick around and, after my instructor patted me on the shoulder and gave me the “OK” signal, headed back up the line for the safety stop and surface.

More light, life and warmth in the 10-to-40-foot range underwater like here at Rachel Carson Salt Pond. Photo by Charles H. Lagerbom

We get some divers who join us who want to do the deeper types of dives, maybe to show bravado or something. For the most part, we tend to stick in the less-than-50-foot range. This is where it is warmer, there is more light and way more marine-life to see. Deep Diving is its own kind of adventure; maybe I’m just more shallow-minded.

Charles Lagerbom teaches AP U.S. History at Belfast Area High School and lives in Northport. He is author of “Whaling in Maine” and “Maine to Cape Horn,” available through Historypress.com.