You know that feeling when you’re blissfuly happy, and you feel super-confident? Maybe you’re in love, or high on something? Well, you can get that feeling easily when scuba-diving underwater. It’s called nitrogen narcosis – the narcs – and it can get you killed…

First the basics. When you’re diving with air in your tank, you don’t normally get it above 30 metres (100 feet). Below that depth, the partial pressure of nitrogen starts to affect our brains, and it’s very much like getting drunk. One of my most memorable dives was diving to 50 metres in Indonesia with a large school of tuna. I had the narcs for sure. and actually felt like I was a tuna. I swam with the school, turned when they turned, and was on the watch-out for sharks. My buddy stayed at 40 metres, watching me (in hindsight, not much of a buddy, no chance of rescuing me if i’d have gone deeper). 
When you get the narcs you believe you can go deeper. Then the narcosis will increase. The happy-go-lucky feeling can then change into panic and disorientation. I used to train divers to show them what it was like. I’d get them to tie a knot called a bowline on land, next to a quarry filled with water. They’d do it easily, without really having to think about it. Then we’d descend to 30 metres – the bottom, as it happens – and after five minutes I’d ask them to tie it again. They couldn’t. Then they’d start laughing, and it would be time to go up. 
If you dive a lot, you get a kind of immunity, as your brain adapts. This is what divers call doing ‘build-up’ dives. If you do a couple of dives at twenty, then twenty-five, then probably 30m will be no problem. But you have to watch the signs. One of the golden rules in diving is ‘Plan the dive, and dive the plan.’ This is because on land you’re stone cold sober, whereas underwater you might decide to change the plan, and maybe it’s not such a good idea. 
There’s even a sign between divers to let someone know you have the narcs. You point your index finger to your temple, and then draw a circle. If your buddy gives you this signal, you should go up. Often you don’t need to go to the surface, maybe ascend just 5 or 10 metres and suddenly your mind will be perfectly clear as again. But if you ignore it. well… It is like getting drunk. There’s a period when you realise you are getting drunk, and have the choice to stop drinking. If you don’t, you tend to think you are no longer drunk, and that you are actually making perfect sense… 
Here’s an extract from 66 metres, where the heroine, Nadia, gets narcosis. She’s an experienced diver, and should know better, but she’s under stress: she has to retrieve an object called the Rose, at 66 metres, or the Russian Mafia will kill her sister…   
Ben shut off the engine completely, the world suddenly silent barring the slip-slops of wavelets against the hull. ‘The prow is directly beneath us, at twenty-five.’

Jake nodded, squirted a little liquid soap into his diving mask, smeared it round with his finger, then leant over the rubber tube and rinsed the mask in seawater. Nadia did the same, looking into the green-blue water below, knowing the Rose was down there, waiting for her. Maybe she could retrieve it today. Why the hell not? Ben opened both their tank valves fully, then she and Jake sat on opposite tubes facing each other, masks on, regulators in their hands.

‘We stop at the prow,’ Jake said. ‘To check we’re both okay. Do you want to go inside the wreck?’

She didn’t. She wanted to plunge straight down to sixty-six, grab the Rose and come back up again. But he’d already said fifty was the limit, and she wasn’t dived up yet, and would get narcosis. Maybe if he went inside then she could go down alone…

‘Sure,’ she said.

‘Then stay close,’ Jake said.

Ben counted down. ‘One, Two… Three!’

She rolled backwards off the boat. The water hit the back of her neck and flushed into her wetsuit, warming up almost instantly. She sucked in a lungful of air and righted herself, and brought her head above the surface. Jake was already next to her. He gave her the OK signal then the thumb-down signal to descend. She returned them both in sequence, then held up her inflate hose and dumped air from her jacket. She sank beneath the water, the last airside view a rippled one of Ben leaning over the side of the boat, watching them disappear.

Jake dove down ahead of her, streamlined, occasionally twisting around effortlessly to check she was following. The water was featureless, and she felt like a parachutist dropping through green-blue sky. Keeping her breathing even, she cleared her ears every five metres or so, and studied his technique: Jake had his arms folded in front of him, the computer on his left wrist so he could read it, his right hand holding the inflate hose, jetting air into his stab jacket every seven or eight metres. Poetry in motion. She adopted the same position.

They fell through sheets of green-blue water fading to grey, the visibility about ten metres, the strong sunlight above gradually leached out by the depth. She couldn’t see anything ahead except Jake. Then a shape emerged, dark, pointed, big. Her heart rate kicked up a notch. Not everyone loved wreck diving. Some preferred ‘scenic’ dives with lots of fish. She didn’t get it. Wrecks were scenic and full of fish.

The prow of the Tsuba loomed out of the grey. A single spotted dogfish patrolled it while a small school of black bream hugged the sloping foredeck. Ben was good, he’d dropped them right on target. Jake slowed. She jetted air into her stab a little late. While he stopped centimetres from the rust-laden prow, hovering as if in space, she rammed it, and had to brace herself against it with outstretched arms. To recover, she let her momentum spin her body and legs around vertically, like a gymnast doing an underwater handspring, so she ended upright, one hand on the prow’s edge, the rest of her body parallel to the deck. As if she’d intended it that way all along.  

Jake gave her an appraising look, followed by the OK signal, which she returned. He then pointed to his air gauge. She looked at hers – one eighty bar – then showed it to him. He returned the favour. His was still at a pressure of two hundred atmospheres. Rule was, you surfaced when it got down to fifty bar, though she’d often left it much later than that. Jake aimed a flat vertical hand down the deck, and she started to descend the ship, tracing its steep seventy degree angle.

Good visibility wasn’t always best for a wreck dive. It was awesome to see an entire wreck underwater, but sometimes poor viz meant discovering a sunken ship bit by bit. The foremast emerged out of the grey. She glided in slow motion over two cargo holds, shining her torch down into them, illuminating a fog of tiny fish in one, rusted spare engine parts in another. The bridge beckoned, four steps and two metal railings inviting her inside. She turned to Jake and he nodded, so she went straight through the open hatch, careful not to bump the ragged metal sides, the rusty edges brilliant shades of orange in her torchlight.

The upper floor had almost completely eroded, so they finned up a few metres, and she found the helm, a classic antique ship’s wheel, most of the wood gone but enough of the brass fittings left to discern its original shape. Like any wreck diver she couldn’t resist grabbing it and staring out towards the mast, just visible. She realised she was grinning, and wondered if it was narcosis setting in. No, she was simply enjoying the dive.

Jake headed aft and she followed, descending deeper into the bowels of the ship.
Her thoughts became sluggish, as if she’d had a few vodkas. She watched his fins undulate in front of her as they entered a narrow black corridor. She could fin faster, show him how Russians dived. Without warning she kicked hard and thrust ahead of him like a torpedo. She misjudged it and her tank grazed the ceiling. She rebounded and ricocheted towards the floor. She let go of her torch to brace herself, and her hands disappeared into a thick layer of sludge coated with powdery sediment that plumed up in front of her mask. She could see nothing, and was still descending. Dammit, the walls were narrowing.

She tried to turn around, banged her head against solid iron. Shit! She couldn’t think straight. Panic rose in her chest, her breathing loud in her ears. Her torch hit the side of her head, its light lost in the black sediment, as if she was in a coal mine. Stop, dammit! Just stop moving. He’ll find you. But where was he? She tried to think. Had she turned left or right? Her breathing rasped ever louder in her ears.

Without warning she was tugged backwards sharply. Jake must have grabbed her fins. His hands pulled her around in the semi-darkness, her torch beam flailing wildly like a beacon in fog, still attached to her stab jacket by its thin cord.

Jake brought her close, right in front of his face, mask to mask. She breathed heavily. He put two straight fingers in front of her eyes, waggled them as if they were walking, shook his head once, then put them tight together, unmoving. She got it. Don’t fin.

He put an arm around her waist, just underneath her stab jacket, and kept eye contact with her. She had to fight her normal instinct to struggle free and be independent, which would only get them both into trouble. She stayed still. Jake edged them back out of the soup, pulling them along with one arm, and suddenly the water cleared, and they were back on the bridge. Her panic vanished. Narcosis was so depth-dependent: one second exhilaration, the next all-out panic, but a few seconds later and ten metres higher and her mind was clear as a bell. Jake studied her, and she nodded as if to say she was fine, gave him a clear OK signal to verify it, and he let her go. She followed him back outside the ship, and checked her air. Ninety bar. She showed it to him. He looked at it but didn’t show her his.

She’d blown it. Not nearly enough air left to go down to the sea floor and start searching, and in any case the narcosis would return straightaway. Fucking hell! The Rose was down there, waiting. She wanted to punch the wreck. But you can’t punch anything seriously underwater. At least she was alive. Next time she might not get narked at all.

Jake moved away from the Tsuba, and she followed him, close to an eel that slithered off in the direction of the underwater pinnacle propping up the wreck. She and Jake slowly ascended amongst lush green ferns, flora she normally spurned. Fish skittered over mossy boulders, and she tried to take her mind off this catastrophic dive. As they rose above the promontory, the prow of the Tsuba loomed into view again. A cuttlefish, changing colour mid-water, calmed her down a little. Her computer said she’d touched forty-eight metres, and required a decompression stop for five minutes, probably more by the time they arrived at six metres.

She gazed down the disappearing length of the ship. The Rose – her and Katya’s key to freedom – was down there, and she’d been less than twenty metres above it. 

It might as well have been a kilometer.

66 Metres available digitally everywhere. Please don’t read your kindle or iPad underwater…