AS YOU DRIVE SOUTH FROM RIYADH TOWARDS AL KHARJ, THE DESERT IS DOTTED WITH CIRCULAR AREAS OF LUSH GREEN GRASS. Its cultivation in the middle of the Saudi Arabian desert is made possible by natural reservoirs of fossil water lying deep underground. In some places the water comes close to the surface and big sinkholes - or dahl, in Arabic - have opened in the ground.
I had been fascinated with these holes for years. I fantasised about a complicated underwater cave system waiting to bediscovered, but all the holes I had seen had 60m vertical walls down to the water - hopeless for getting in with scuba gear.
Then I heard about Ain Hith, in the Sulaiy escarpment. A wall of limestone is interrupted by a huge cave, the Dahl Hith, which leads 100m down to an underground lake.
I was told that a diver, using only a mask and fins, had lost his way inside the underwater cavern and drowned. As far as I knew nobody had tried exploring it using breathing equipment, so I planned an expedition that led to two years of hard and dangerous work, with at least one close call on my life.
The cave begins with a huge opening in the porous limestone that makes up the Sulaiy escarpment. At the base of the escarpment is a blue-grey coloured anhydrite (calcium sulphate) formation.
Water has been infiltrating into the subsurface here for tens of thousands of years, dissolving the highly soluble anhydrite and forming deep chimneys. Groundwater flow has led to horizontal expansion of the cavities along joints or fractures.
Anhydrite and another calcium sulphate mineral, gypsum, have precipitated in layers on top of the limestone base as highly mineral-charged ocean brines have evaporated.
The anhydrite layer, laid down 140 million years ago, served as a massive lid on the oil reserves that were eventually found. According to legend, in 1938 a group of US oil explorers came to visit King Abdul Azis in Riyadh. After five years of unsuccessful drilling in Dammam, they were about to give up when the king took them to the Ain Hith waterhole for a picnic. When the engineers saw the structure of the exposed ground layers, they realised that they might need to drill deeper to reach the oil.
They went back to Dammam, got to work, and struck lucky.
The cave system is divided into three separate chambers. The first cave is lit by sunlight and runs down at a 45Â angle. It is not a difficult climb but it can be hard work with diving equipment on your back, and the floor is covered with sharp limestone boulders.
To reach the second chamber, we had to crawl through a small opening. The dive equipment had to be lowered with ropes and, as the sunlight disappeared, we resorted to torches.
The chamber measures about 25m by 10m, and is 10m high. From here a larger opening leads into the third chamber. This space, with the lake at the bottom, is about three times the size of the second one. I had never seen such clear water. The surface was so still that it was hard to see where the water level began, and light refraction made it look far shallower than it actually was. The bottom shimmered light green, and we could see a black hole forming the entrance to an underwater cave.
After rigging gaslights to illuminate the cave, we put on our diving equipment. We decided to dive in pairs, with one diver connected to land by a lifeline, and we brought weighted chemical lightsticks to mark the way. Both divers had powerful torches and smaller back-up lights.
We had no idea what lay before us. Would we end up in a current and be swept away I instructed the lifeline-holder to keep the rope tight at all times so that we could have signal contact with him.
With my heavy diving and photographic equipment, it was tricky getting down the few steps to the water, but soon we were on our way. We went over a ledge and down into a big underwater cave with crystal-clear water. With our torches we could illuminate the entire chamber. The bottom was covered with a very fine silt of dissolved anhydrite that looked like flour. When stirred up, the water resembled milk.
In the silt I could see the glitter of beautiful gypsum crystals. We put out the lightsticks to mark the entrance and went on. The cave forked in two directions. Straight ahead it narrowed into a black tunnel, and below it fell away in a narrowing canyon. We decided to explore the forward route first. My partner led with the lifeline; I followed with a hand on the rope.
The shapes of the walls were surreal. They were covered with white silt as if it had been snowing, and when our exhalation bubbles struck the roof, flakes of loose rock came falling down like snow. We had to move very carefully, gliding through the water with almost no movement. Neutral buoyancy was imperative.
The tunnel narrowed. Finally, after swimming about 150m, we struck the roof and came to a dead end. We turned back. The line-holder was doing his job well and we had no problem following the rope as we retreated, so we decided to see if the canyon below led any further.
We swam down in a zigzag course, avoiding the silt-covered shelves. We saw several side-tunnels leading in other directions, but followed the main shaft. It became increasingly narrow until, at last, we reached an opening just big enough for one person to pass through.
I looked at my depth gauge - 17m - then glanced back the way we had come. I could see that the rope had snagged the rock in several places, which meant that we no longer had contact with the line-holder. To make matters worse, we had kicked up silt, which now came rolling down towards us like fog.
I began to feel uneasy. If we lost our visibility now, we would be in danger. My partner was relatively inexperienced and seemed unaware of our situation. Things rapidly went from bad to worse, but before I could signal to him to turn back, he started crawling through the opening, kicking violently with his fins.
In an instant, all visibility had gone. We were left groping in a milky soup and there was no way of judging direction. Only the rope leading upwards, white in the torch-beam, offered a chance of returning safely.
Desperately signalling to my partner to turn back, I began a slow ascent, following the rope hand over hand.
I saw the gleam of my partners torch and knew he was following. Fear started to slow down my thinking - I knew I had plenty of air left but felt I was not getting enough. I started to breathe heavily and had to lie down to control myself before doing anything irrational, like going for an uncontrolled ascent.
I came to a rock where the rope was jammed. I put the light aside and blindly loosened the rope before going on. After several similar stops, the rope was free. I was back in touch with the line-holder and my feeling of security returned.
Soon we found the lightsticks marking the tunnel entrance. We were shaken, and I was glad to see the sun again after an exhausting climb back to the main entrance.
Before we had turned back, my partner had looked through the hole and seen that it went further. It was an irresistible challenge to find out what was on the other side, but I realised we would need more expertise and back-up equipment next time. I was now entering the realms of serious cave diving.
A few weeks later, I received a telephone call from an experienced cave-diver, Mike Gibson. He had just arrived in Saudi Arabia and, on the remote chance that he would do a bit of cave diving, had brought all his advanced equipment!
We went back to the cave with Arlene Foss, a diver who accompanied me on my shark expeditions. Mike went down to secure a permanent lead rope through the passage where we had turned back. He took two independent breathing systems to give him a backup in case of equipment failure.
He had two lights fixed to a helmet, a main halogen light and two reels with rope - one for the main guideline and one to use for any detours from the main shaft. He went away and came back 45 minutes later, bubbling with excitement, to an eager surface party. There was a continuation, and it was big.
The following week, Arlene and I went down. We headed straight for the entrance to the second underwater chamber. Mike had tied the rope to rocks so that it rested a foot above the silt. Arlene could not avoid stirring up the bottom in the narrow tunnel near the cave floor, so I had to go through in zero visibility.
I pulled myself through carefully, but with my tanks hitting the roof. On the other side, my light revealed a magnificent sight. A dome-shaped cave opened in front of me, and Arlene was hovering under the roof. The water was transparent and the light spread through the whole cave. It was like hovering weightless under the roof of a cathedral.
The white silt layer gave the floor soft, rounded contours; the walls and roof had shelves with sharp edges. I looked into a small hole in the wall and it was filled with the most beautiful small crystals, like an ice cave.
From the entrance hole there was a downward slope. I could see another tunnel opening 50m further on, with tiny tracks in the silt like those insects make in desert sand. Like the shoeprint of an astronaut on the moon, they stayed undisturbed until covered with more silt.
I soon found the origin of the tracks - tiny, transparent, shrimp-like creatures. In the light I could see their red haemoglobin. They were amphipods, a kind of crustacean, but we could not identify the species. They had to be blind as there was no light in the cave, and considering the isolation of the place, they might very well have evolved there. If so, they could be unknown to science.
How could life exist where there was no sun Perhaps nutrients were transported from areas exposed to sunlight, or perhaps these creatures were at the top of a foodchain that started with bacteria drawing energy from chemicals such as sulphur and hydrogen - similar to the lifeforms recently discovered on the ocean floor.
I continued down the slope, following the rope, and went through the next opening at 26m. I came to a 90Â bend and entered a smaller chamber that had two tunnels running straight ahead and left. I went straight and came to an area where the roof was lower and the cave extended horizontally in all directions.
It was formed by big bubbles in the limestone, which meant that the cave was not the work of running water but of slow dissolution of the rock.
It looked like a mosque, with pillars and arches. The rope ended here, so I went around the perimeter of this chamber and saw a narrow tunnel going right. By now we were far inside the cave system, 30m deep. It was too big a risk to go further.
We were on our way back when an eerie rumbling and echoing sound like falling rocks made my blood freeze. I waited for the roof to collapse over me. Then I looked up, and realised that we were all right. The roof was dotted with pits that had filled up with our breathing air on the way in. When we turned back, the air bubbles had started running along the roof to higher areas, creating the sound.
We continue to explore the Ain Hith cave system. It is a tediously slow task, because we can do only one dive on each visit before the cave is silted up and visibility gone. To climb down and back from the water in sometimes 45ÂC heat with 20kg on your back is a real test of endurance.
When I go through the small entrance to the inner cave at Dahl Hith, its like stepping down from a spacecraft onto the moon. You hover weightless in a place with a surreal landscape where nobody has been before.
There is always a certain amount of anxiety about what could go wrong, but the sirens keep singing their song to draw us in further.

Well into the large second chamber the last sunlight disappeared
Mike Gibson and Erik Bjurstrom
in the first underwater chamber the bottom was covered by a fine silt
the tunnel leading to the dome
the dome-shaped cave opens up
passing through an opening at 26m
lan and section of Dahl Hith, showing the descent to the Mosque