Imagine some of the most physically demanding jobs available—supply line installation, construction, welding—and then imagine doing them underwater. That’s the life of a commercial diver, a rigorously trained professional who undertakes everything from bridge repairs to oil line maintenance. To get a better sense of this often difficult and dangerous work, Mental Floss spoke to several commercial divers for their thoughts on everything from the perils of decompression to swimming in sewage. Here’s what they had to say about a life in flippers.

1. DIVING DEEP CAN PRODUCE EUPHORIA (AND A WEIRD VOICE).

Commercial divers receive specialized training—either in the military or at diving instructional schools—to learn how to function hundreds of feet below the surface. The lower a diver goes, the more water pressure increases, and the greater the challenges. Jeremy, a commercial diver out of Louisiana who repairs and installs equipment for oil companies, says that working in such conditions can lead to physical exhaustion, pulled muscles, and a feeling of pressure on the lungs.

Plunging to a depth in excess of 100 feet can also result in nitrogen narcosis, which some refer to as "raptures of the deep" or the "Martini effect." It's caused when divers receive a higher concentration of nitrogen from their air supply due to the effects of the water pressure on the gas. (The air systems that commercial divers use allow them to breathe normally by providing air at a pressure equal to that of the water, but the lower they go, the denser the gas gets, and thus the higher the concentration.)

“It makes you feel drunk or euphoric,” Jeremy says of the narcosis. “The solution is to switch from a nitrogen-oxygen supply to helium and oxygen.” That cures the over-inhalation of nitrogen, but when a diver comes back to the surface or to a decompression chamber, their voice will be altered. “It’s an Alvin and the Chipmunks thing,” Jeremy explains. Some diving teams will use voice augmentation to de-scramble the high-pitched squeals when divers are communicating with the surface.

2. ABOUT HALF A DOZEN OF THEM DIE EACH YEAR.

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Most commercial diving is centered around underwater construction—often repairing or replacing infrastructure that facilitates water, oil, or electrical supplies. Divers are frequently charged with digging trenches to bury electrical lines using high-pressure water blasts to excavate the ocean floor. If these trenches collapse, it can result in a catastrophic situation; the cave-in can trap and bury a diver, clogging their regulator or causing them to take off their helmet in a panic, which eliminates their air supply. Jeremy says a number of divers die every year in such cave-ins.

If divers can avoid that fate, they still have to worry about a number of other ways they can meet an untimely end. “We use cranes and those can fall or drop their load on you,” Jeremy says. Cutting into “live” pipelines can also cause explosions, as can using tools that displace hydrogen from the water. In an enclosed space like a ship or supply pipe, that collected hydrogen could catch a spark and explode. “That could blow your helmet off or into pieces,” he says. All in all, 25 commercial divers died on the job between 2011 and 2014, according to the Bureau of Labor Statistics; another 310 suffered nonfatal injuries or illnesses.

3. THE DEEPER THEY GO, THE MORE THEY EARN.

Diving jobs vary in pay according to risk, duration, and other variables, but generally, a diver’s base pay is usually supplemented with “depth pay.” The further down they go, the more they can make.

“It’s basically about a dollar a foot,” Jeremy says. “After 150 feet, the price can double to $2 a foot. Added on to regular pay, a 12-hour day can add up.” A diver working at 300 feet might net $1000 in a shift. Saturation divers, who can go 1000 feet down and are required to live off-shift in a chamber pressurized to the surrounding water in order to avoid decompression sickness, or the “bends,” can make even more.

4. SOMETIMES THEIR SUITS ARE HEATED.

Going deeper into the water means enduring more frigid conditions. To offset plummeting temperatures, divers need a way to keep their suits warm. “Below 80 feet, it gets cold,” Jeremy says. “We either pump water into a wet suit or wear a hot-water suit.” The former allows water to come in and make contact with the diver's body, typically from a heated source at the surface; the latter has water channels throughout the suit that branch out and keep divers from getting too cold. Because hot water suits can maintain a more consistent temperature than delivering warm water from above, they are most often used at 200 feet and lower depths.

5. THEY CAN WIELD FIRE UNDERWATER.

Most tools meant for underwater use are hydraulic (involving the use of water or other liquids), since they’re largely unaffected by water pressure. Fuel-powered or pneumatic tools (those that involve the use of gas) don’t really work, but divers can still make use of jackhammers, chainsaws, and other devices you’d find in an above-ground construction job. Others, however, need to be adapted.

“In my opinion, the most interesting adaptation is the BROCO torch,” says Brian, a diver based in New England. The BROCO torch uses direct current to ignite a magnesium rod and oxygen mixture that burns at approximately 10,000 degrees and can cut through metal like butter, even underwater. (A/C, or alternating current, is what we use in our homes—but because the direction of the current reverses many times a second, Brian explains, it can freeze the diver in place while electrocuting them, making it too dangerous for underwater use.)

6. THEY MIGHT FIND DEAD BODIES.

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According to Jeremy, many recovery dives for people suspected of drowning fall under the purview of local law enforcement. Still, commercial divers can encounter someone who’s wound up in a watery grave. “I’ve done helicopter recovery jobs,” he says, referring to crashed aircraft that can harbor passengers. Once, while working on an oil rig, he stumbled upon a dead scuba diver. “It was more of a skeleton in a scuba suit,” he says. If a diver does find a corpse, they're unlikely to ever know the history of how the body got there; such discoveries are required to be passed on to the Coast Guard for investigation.

7. THEY CAN WIND UP FEEDING FISH.

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“We encounter marine life all the time,” says Mike, a commercial diver who now works primarily in and around the Great Lakes. “When working the ocean, if we are cleaning off marine growth, sometimes you will get some fish that come up and eat what you are cleaning off.” Mike says that commercial divers frequently spot sharks, barracudas, and other potentially dangerous sea dwellers, but the animals generally don't care much about humans. They’re even less likely to approach if the workers are using torches.

8. THEY SOMETIMES SWIM IN UTTER FILTH …

A common component of commercial diving, HAZMAT (hazardous material) diving involves working in contaminated water. That could mean anything from a lake affected by nearby lawn chemicals to checking equipment at a nuclear reactor. If it could kill or poison you, a diver has probably swum in it.

This kind of work requires a special approach. Brian says that those who venture into higher-risk HAZMAT diving usually wear a positive pressure diving helmet; since the pressure inside the helmet is greater than the pressure in the water outside, the helmet helps keep hazardous material from entering. HAZMAT divers also wear a rubber dry suit that fully seals the diver's entire body, unlike normal wet suits, which allow water to make contact with the wearer. Support staff will also decontaminate the HAZMAT diver after the job, scrubbing their suit free of harmful materials before the diver undresses.

9. … INCLUDING SEWAGE.

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Those stories you may have heard about people diving into sewage treatment plants to repair equipment? Those would be commercial divers, who occasionally brave the psychological challenge of being submerged in poop. Because it's usually impossible to see in a sea of feces, divers will study reference photos of empty tanks before going in. They'll suit up in sealed dry suits and typically will weigh themselves down in order to sink through the dense liquid; once they're in position, they work by feel. “Both the sewage jobs I dove on, it was repairing a masticator blade,” Mike says. “Picture a giant blender that makes solids less solid. I don't do it anymore because of the health risks.” A rip or tear in a diver's suit can introduce a litany of dangerous bacteria into their body: In addition to your standard Salmonella and Cryptosporidium parasites, such vile muck can also harbor hepatitis, Norwalk virus, E. coli, and assorted fungi [PDF].

10. DAWN SOAP IS A LIFESAVER.

Dawn dishwashing liquid is a must-have on diving expeditions. It can get diving suits and skin free of oil, and can even help divers cope with parasitic pests. When Jeremy was working on a mile-long pipeline near New Orleans, the shallow water resulted in workers getting infested with parasites carried by nutria, a semiaquatic rodent. “The hookworms will dig into your skin, die, and leave a big red mark,” he says. Splashing Dawn soap gets rid of the itch immediately. (If irritation persists, divers might need to seek anti-inflammatory treatment from a dermatologist.)

11. THEY WORRY ABOUT BEING SUCKED INTO A VACUUM OF DEATH.

Divers are frequently in violation of the laws of nature. Humans, after all, were never meant to thrive (or survive) underwater, particularly at more pressurized depths. Many divers fear encountering Delta P, or differential pressure—a vacuum that’s far higher in pressure than their current environment, and is created by intersecting water bodies as a result of opening a channel like a pipe. “Delta P is vacuum-like suction much like you would imagine from when the cabin of an airplane ruptures, but at a much greater magnitude,” Brian says. “It can be very difficult to detect until you are already too close, and can trap the diver at depth or even kill them instantly.” The unfortunate crab in the video above is an example of how differential pressure can ruin your day.

12. THEY SOMETIMES GO DIVING INSIDE WATER TOWERS.

Those water towers you see in populated areas that stand on stilts hundreds of feet up in the air? Townships need to periodically check them for sediment levels to maintain water quality. That’s when they call in a commercial diver, who needs to add "not afraid of heights" to their skill set. “You have to climb all the way up, get into your wet suit, measure the sediment with a ruler, and clear it out with a [suctioning device called an] airlift." Jeremy says. And that's not the only lofty prospect for a diver: Jeremy notes that some oil rigs stretch 100 feet in the air. Divers without seniority may be expected to carry out repairs or work at or near the top, instead of actually diving.

13. THEY CARRY KNIVES.

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No, it’s not to duel with sharks. “While diving, I carry a razor-sharp knife for emergency purposes only,” Brian says. In an urgent situation, it could be used for "cutting anything from old fishing line to my own dive umbilical—the air hose and lifeline.” The latter rarely happens, unless the diver gets it snagged or it becomes compressed. In the event of a hose failure, divers have a "bailout bottle," a supplemental tank they can switch to in case of emergency.

14. THEY CAN BE UNDERWATER BUT NOT ACTUALLY IN THE WATER.

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Not every dive requires divers to swim while working. For jobs that require meticulous attention to detail for repair or where welding is required, diving teams can set up positive pressure habitats that isolate the problem area and allow the diver to work out of water. “You use air pressure to push water out of the habitat, which is in two pieces,” Jeremy says. Inside, a diver would trade their helmet for a welding mask. Because it can take a day or more to set up the habitat for a job that might take only one or two hours, habitat work is used only in cases where there aren't any other options.

15. THEY STILL GO SWIMMING FOR FUN.

Like anything done recreationally, diving can begin to seem routine if it's performed on a daily basis. While some divers get their fill of water by working 12-hour days for weeks at a stretch, some still enjoy going under in their free time. “While my career has definitely diminished the novelty of being in such an alien environment, I still love to dive recreationally,” Brian says. “Commercial diving is exhausting work, typically in dark, low-visibility water with a particular task in mind, while recreational diving is often more about exploration and sight-seeing. I would argue that the difference is not unlike a professional runner going on a beautiful hike in their free time.”