My interest in this area began whilst sitting next to the boat captain as he cursed the echo sounder's ‘off the scale' display, "Must be a bad wiring connection" he muttered, although it was strange how the sounder always started working again shortly after. For sure, the water at the ammo dump was deep…Very deep, probably chosen as a deterrent against local air divers from bringing up the explosives up to 'go fishing' with.
Dives into the Samesan ammo dump area (part of a busy inshore shipping lane) always require careful planning due to the very strong currents, surface traffic including large and fast oil tankers and depths well in excess of 80 metres. Oh, I almost forgot the whirlpools, updraft and downdraft currents that send your exhaled air bubbles downwards out of sight while hanging on to the descent line. In other words, a perfect technical diving training site! Challenging as this dive site is, with careful surface cover to dispatch oncoming supertankers away from decompressing divers, it is usually survivable, But I had long been on the lookout for something a little less hair raising, with less risk of being sucked off back to depth by whirlpools, chopped up into small pieces by oncoming supertankers, or more likely whisked off mid-ocean and 'lost at sea' which has already happened during a deco stop.

Mermaids Dive Center, a leading technical training center in Thailand, began searching for a safer location for Trimix diving in the 50-90 metre range.

A large area of seabed around the Samesan area was sounded with new, more powerful sonar, revealing what appeared to be an uncharted underwater cliff off another popular dive site called ‘Shark fin Rock'. At around 87 meters (280ft), out of the shipping lanes, and in an area of mild current, the new location was chosen as the site for the final qualifying dive for Thailand's second Advanced Trimix course.

Divers in the team, led by Mark Ellyatt from Atlantis Divers in the Philippines, were Lars Steffensen, diving a ‘Buddy Inspiration', followed by Andrew Yates and myself on Open Circuit Trimix.
Preparations went on into the night, and finally a 14/45 Trimix was settled on with Nitrox 40 and pure Oxygen for accelerated decompression.
Conditions at the dive site, which Mark had by now started referring to as the ‘big black hole of death', were calm and the viz was good. It seemed surprisingly survivable.

A large barrel sponge and a big gray stingray met us at the undersea landing spot, it's perfect camouflage unfortunately leading me to mistake it for the seabed land as I landed firmly on it with 100Kg of 4 tank tech rig pinning both wings down. A short circular search on the reel for other marine life unfortunately only revealed a pitch-black cloud of zero-viz silt of our own making.
The only casualties of the dive were 3 locked out Aladdin Pro's, a Suunto in a strange Error Mode, a slightly bruised ray, and a dive boat captain complaining of pulling his heavy primary anchor up by hand from nearly 300ft down. I'll get him a big lift bag for next time!

Safety Note:
The diving to depths outside recreational scuba limits must only be carried out following a formal course in technical diver training. For safety and physiological reasons, the use of Tri-mixtures or Heliox is recommended for depths below 40 metres. The use of 45% Helium in the bottom mix of this dive together with only 14% Oxygen (a hypoxic mix) allowed the divers to retain a clear head at depth together with Oxygen partial pressures within normal physiological limits. The use of air as a breathing mixture at these depths would likely result in fatal consequences. Any ‘air' dive computers carried on this dive were used for depth and time indication only, and it is normal for them to ‘lock out' and not give any sensible deco information when carried on deep, accelerated deco dives of this type. Decompression was carried out entirely by ‘computer cut' tables written on wrist slates.

Training
Mermaids Dive Center now conducts Advanced Trimix dive instruction down to 90 metres (290ft) together with trips out to the some rarely dived deep shipwrecks in the Gulf of Thailand as part of a comprehensive program in Technical Diver education.