The Lagoon Lost in Time, high in Venezuelas Andes Mountains

The lake was reckoned to be about 60m deep, at an altitude of 3800m and nearly 10 miles from the nearest road. It would take five hours to get there on foot, ascending over a small mountain range.
The Venezuelan freediver Carlos Coste is current world record-holder in both the Constant and Variable Weight disciplines, so he knows what hes doing. Even so, his latest venture sounded like a great challenge.
My only experience of diving at altitude had been on Bodmin Moor, in a small quarry called Gold Diggers - 300m high.
In Venezuelas capital, Caracas, it took us three days of paperwork and battles with bureaucracy before we were able to depart. The eight of us finally crammed our kit into two 4x4s, and we were relieved to take off.
Caracas had been dirty and full of poverty, crime and pollution, but we were soon surrounded by beautiful Venezuelan countryside.

The journey
After seven hours, we left the flat plains to begin our three-hour ascent of the winding mountain roads. At around 2000m we entered Merida state, gateway to the Andes Mountains.
We spent the night at a small villa at the foot of a huge mountain, to acclimatise and organise our equipment for the next days hike.
The first signs of altitude sickness soon appeared. Dyspnea, or shortness of breath, came without warning and proved unpleasant, especially through the night.
This was quite normal, I was told, and would wear off after a few days.
Altitude above 2000m means a lower partial pressure of oxygen, which causes arterial hypoxaemia.
In response to hypoxic stimulation, hyperventilation occurs, with secondary lowering of arterial CO2 and production of alkalosis.
Thats why visitors are usually advised to spend at least two or three days acclimatising before attempting anything too challenging.
But in the morning we headed up to the meeting point, to find our team of 24 mules and their sherpas. After an hour of loading, we set off. The local sherpas seemed to be breathing OK,
but within minutes I was out of breath, and had to stop. I was still at only 3500m, with a five-hour trek ahead.
After a while, I set my pace and tried to switch off, my mind fixed on the top on the highest peak, at 4200m. My breathing was rapid; my heart racing. Each mule carried 80kg of gear and supplies. My large white animal carried my two sets of double 15s, and looked unimpressed. I avoided eye contact.
After three hours, we reached the highest point of the journey, and were greeted by some of the sherpas wives with a meal of rice, chicken and veg. The views were wonderful, and we knew it was all downhill now.
Two hours later, we arrived at the edge of something from a movie, The Lagoon Lost in Time, perhaps. It had its own microclimate and produced its own small clouds, which floated mysteriously above it.
The campsite had already been assembled by the sherpas, who had arrived an hour before us and also prepared a hot meal. The sun dropped behind the mountains, and the temperature fell rapidly. We wrapped up in balaclavas and our thickest thermals, and drank tea as the temperature plummeted.
By 10pm it was below freezing, and we headed to bed. Unfortunately the sleeping bag with which I had been provided was too small to cover a good third of me. It was made for a short Andean guide.
I lay shivering till I finally passed out. Warm sun woke me at 6:30am.

Training day
The ground was still frozen around our tents, but once the sun hit the lagoon our enthusiasm returned and we started to set up the equipment for the first training dive. We sent out a team with one of the small inflatable boats to scope the unsurveyed lake and find the deepest part.
I had done my best to research high-altitude diving before leaving Venezuela, but had found little information on dives above 3000m.
I had found some material on the Internet, and in John Lippmanns book Deeper Into Diving. But no relevant tables existed, so I planned to use some US Navy tables for a back-up schedule.
By my reckoning, my equivalent ocean depth for a normal 51m dive would be 85m, so I planned the first dive with the next greatest depth, and added extra safety.
The nearest hyperbaric chamber was, after all, more than 15 hours away.
I took a VR3 as my primary computer and a Suunto D9 computer as a back-up depth gauge.
The VR3 was simple to calibrate at altitude, and automatically calibrated to fresh water. The menu gave accurate measurements of altitude and barometric pressure.
I prepared to make a shallow check-dive, and the sherpas and locals stood around in amazement. I realised that they had never seen a scuba diver before.
I dropped through the green gloom to 10m, hovering over a thick layer of silt.
I did a final check, and headed off into the depths, reaching 30m according to my VR3. The D9 read 25m, with a bottom temperature of 10C.
I was at depth only briefly, yet my computer no-stop time dropped rapidly to only a few minutes. I didnt want to go into deco, because a dive with Carlos was planned for the afternoon.
As I glided back up the slope, a huge trout of 7kg or more swam past me. The locals later told me that fish of twice that size lived in the lake!
The survey team had meanwhile located a possible point for what would be an informal (that is, not officiated) record dive attempt, having recorded 56-60m there. A line and shot were positioned for the big dive the next day.
In the afternoon Carlos did his warm-up freedives, and I supported him down to 30m.
He did a few 30m dives, and two deeper dives to 41m, and he reported unusual problems with his ears - perhaps as a result of the cold or darkness, perhaps because of the unfamiliar surroundings.
He had only one day to do his big dive, and this was the coldest water he had ever experienced.

Record bid
The day was sunny; the lake was still.
I kitted up my doubles in a 2.5m boat, and sat at the surface as Carlos made a few shallow practice dives.
The 10-minute countdown began. The other safety diver and I were ready, Carlos was ready - the five-minute top time was announced.
I grabbed my video, and descended to the bottom of the lake, swapping the green gloom for darkness as I sank. At 40m, it was black. I dropped to 50m and stopped. All I could see was the yellow line, and nothing below me.
The lake was still and dark. Within a few minutes, I heard a safety karabiner sliding down the line. A silhouette appeared, and I saw a flash of silver. Carlos freefell to the bottom plate, touched it and, with a powerful kick on his monofin, took off back up the line.
After my deco, I popped up to the surface to find everyone celebrating.
Six months of planning and training, and it was all over in just a few minutes. Carlos Coste had conquered the Andes, and his high-altitude dive had been a phenomenal achievement.
What next Deep lakes at altitude are few and far between, but our next project involves a lake at 5200m that is more than 70m deep. This would be an extreme undertaking for both freediver and diver. Watch this space.

Carlos Coste launches himself into the lagoon, monofin first.
The Silver Diver celebrates after his extraordinary dive.
The longest climb - a longer period acclimatising would have benefited newcomers to the Andes like Dan Burton