THUD... OUCH!!! (expletives!) - forward motion is dramatically slowed. Like some boxer reeling after a heavy blow to the head, I flex my neck slowly, and wonder about whiplash.
The vis is about 1.5m here, and Ive crashed into a rock hanging from the roof of the cave. My head hurts.
Thank God for the helmet. The scooter is fine, and I thank my lucky stars that no one else has seen this.
I vow to try to miss that rock in future.
This is Sump 1 in Pozo Azul, (pronounced potho athool, it means blue hole). Once the longest cave dive in Spain, its now prime contender for the longest cave dive in the world.
Its a truly beautiful place, secreted close to the small village of Covanera, 60 miles south of Santander and 30 miles from Burgos. A week down the line and this, the most audacious expedition in the history of cave-diving, is about to witness the moment of truth.
Ive been exploring caves all my life, but never at this technical level. Day after day a massive team of Brits, Spanish and Dutch activists have been beavering away, putting everything in place to mount an exploration akin to a NASA space mission.
I look to history for comparisons.
In the early 1900s it was the race to the South Pole; in 1953, the conquest of Everest. The quest I am witnessing is the ultimate, and the guys I am supporting are the best. Like Amundsen in 1911 and Hilary/Tensing in 1953, they are supremely competent.
To meet Jason Mallinson, expedition leader, Rick Stanton, John Volanthen and Dutchman Rene Houben, you would never imagine that these guys do what they do. They are quiet and unassuming. They are like icebergs; above water you see just a minute part; below the surface there is unfathomable strength and resolve.
Experience is everything at this level, but after 40 years activity in this sphere I struggle to contemplate what these guys are about to do.

I CRUISE ON THROUGH the murky water and reflect on the mission, the vast preparations
and all the uncertainties involved.
Two months ago, we were all in the south of France, where Rick, Jason, John and Rene were heading up another monumental project at the Doux de Coly cave near the city of Brive. It had taken months to prepare for that expedition, and after many days set-up the lads had to sit back and admit defeat.
Storm water, from a month or so previously, had chosen that period to flow through the cave and reduce visibility to barely a couple of metres.
It was bad at the time, but all good experience - psychological and physical - for Pozo Azul.
Now, the goal is the exploration of the ongoing, uncharted, passage 6km in from the cave entrance. Only one person, Rick Stanton, has ever seen that passage, in 2009. Working as part of a close-knit team, his push had been the third, final, advance on that trip. He had been preceded by John and Rene, who had advanced from 4020m in Sump 2 to reach 4395m, and then Jason, again leading the expedition, who pushed on a further 625m to reach the 5020m mark.
Finally, several days later, Rick took his turn, passing the sump just 140m further, at 5160m - the longest sump to be passed in the world.
The small area of dry cave beyond was named Tipperary. Rick had made a brief, 160m, foray into the next flooded section, and established that the cave continued into the unknown. In 2009, therefore, it would take 6020m of diving just to reach the furthest known point.

ONE YEAR LATER, in September 2010, were back, and the massive project is well underway. In essence, this operation breaks down into three stages.
While the diving begins out in the sunshine at the head of a tranquil valley, an advance dive base has to be established in a small dry chamber 1km into the cave. The largest team of cave-divers ever assembled is now plying this section of waterway daily, ferrying load after load to furnish the assault on Sump 2.
Even Sump 1 is, by normal cave-diving standards, a long one. A phone line has been installed, and transporting everything to Sump 2 is no mean feat. Thirty-litre O2 bottles are very heavy, and barely carriable above water; moving them 950m under water takes time and requires considerable flotation.
Four underwater habitats are established a short distance into Sump 2. Beyond these, a large stockpile of bail-out cylinders and other equipment is set along the route for 2km into Sump 2.
Sump 2, at 5km, and 70m deep for much of its length, is a monumental challenge in its own right.
To go even further and tackle Sump 3 effectively requires that the divers camp for two nights in the small dry section, Tipperary. They need to camp to be able to rest, off-gas and, most importantly,
to replenish their rebreathers after the long journey into the cave.
So the lads have to take pressure-proof dry tubes containing sleeping bags, beds, or hammock, stove, food and so on.
Each individual has to be completely self-supporting and to carry his own dry tube. Any leakage or other problem will necessitate a premature exit, perhaps incurring a dive of 12 hours in Sump 2.
In addition to the dry tube, each diver has to take three scooters and a line reel, never mind the 20-litre cylinders of bail-out gas. The logistics are mind-boggling. Nothing can afford to fail, and no one dare suffer any physical or diving-related problem. Rescue is not on the agenda, because there simply is no one to call upon - we dont talk about this.
The one thing we do discuss is the subject of CO2 build-up in Tipperary. The area is of limited size, and the presence of four people over an extended period might result in a real problem with air quality. Such is the concern with this aspect of the operation that Rick is taking a portable CO2 analyser to monitor the situation.
Today is D-Day. Everyone is up before daylight, and we are diving by 8.30. Because of all the diver traffic, Sump 1 is heavily silted, but Sump 2 is crystal-clear.
Spirits are high and all four explorers are composed. If things run to plan (and there is so much that could go wrong!) we know what will follow.
The guys will be out of contact beyond Sump 2 for at least two nights.
The set-up has gone so well, and the atmosphere is so positive, that I get the perfect photo-shoot of the departure. Then we wait.

WITH THEIR STAGGERED DEPARTURE into Sump 2, the guys decompress two hours later at the far, Tipperary, side of the sump without crowding each other. Then they set about organising themselves.
Rene makes an exploratory dive into Sump 3. Perhaps there is a bigger section of dry cave within 500m or so, which would allow everyone to move forward to a better site for camping. This would remove the concern hanging over them regarding their atmospheric air in Tipperary.
In the event, there is no other air chamber, and Rene returns having laid out a whole 1km of line.
The others look at each other, and each is thinking the same thing: Well, thats set the benchmark!
Hours later, with Jason and Rene in hammocks and Rick and John on the floor, the guys settle to a well-
earned rest.
The next morning, John takes his solo turn at the front. He follows Renes line and lays another 125m before discovering line laid the previous year.
Like all caves, Pozo Azul is complex. Rene had moved forward for 280m the previous day, but then unknowingly must have looped off into a side passage.
John quickly back-tracks, locates the main upstream continuation and lays 875m on into the mountain. His furthest point is a very respectable 1315m from Sump 3 dive base, and he too returns
to Tipperary with all 1km of line laid.
The cave is running at about 30-40m depth, and John describes the place as being altogether larger, perhaps twice the size of Sump 1. I just pointed my scooter towards the blackest bit ahead and kept going, he tells the others.
Rick and Jason dive together. They have done this so often that they trust and understand one another perfectly. At the 1315m mark, Rick ties on a fresh reel, and Jason scooters off ahead to scout the route.
In the crystal water, this allows the man running the reel something of a beacon for which to head. This technique ensures that the reel man is following the main tunnel, and not accidentally turning into a side passage or time-wasting alcove.
After 700m, they swap roles. Diving 20-40m apart, they cruise on down the immense tunnel. We just had to keep going, says Rick, it was there for the taking. The cave just goes on and on, meandering as before, but trending as ever in a steady easterly direction. In the event the duo make a five-hour dive, of which two hours are decompression.
By the time they regain surface in Tipperary, they have taken the furthest point of penetration in Sump 3 to 2.8km. In terms of diving distance from the entrance of the cave (that is, excluding dry sections), they have reached 8825m.
One might expect tremendous jubilation at this point, but in fact the atmosphere is subdued. This is no time for celebration. Yes, were safe in Tipperary, say John, but the simple fact is that we have to dive to get out.
Its over 6km to get out from here, and once more each diver looks to his preparations to ensure a safe exit. Only then can they all afford to relax.
The guys are fully aware that the furthest point of penetration in any cave dive previously was that at Wakulla Springs in north Florida, a record of 7.8km set in 2009.
Today they have been a kilometre further than that!

AT 1.19PM THE NEXT DAY, the phone rings at the surface. The underground team waiting at Sump 2 inform us that the first pair, John and Rene, are safely back at decompression.
And when the news comes through that all are safe, and of the incredible amount of line that has been laid, the only words to be heard are staggering and mind-boggling.
Making such an advance just doesnt usually happen, anywhere.
At 4pm, after 50-odd hours in the cave, the bleary-eyed team emerge to the late afternoon sunshine. Its all over.
The expedition is noteworthy on another count. A new mapping device designed and developed by John Volanthen received its first serious trial.
On such long, deep diving operations, it is impractical time-wise to chart distance, depth and accurate bearings. The morning after surfacing, John shows us the fruits of his endeavours, and everyone present is extremely impressed.
The teamwork and camaraderie evident throughout the two-week expedition has been flawless.
While the British and Dutch team-members take the limelight, the operation would have been impossible without the selfless, indefatigable support given by Xesus Manteca and his band of smiling divers, drawn from all parts of Spain.
The team particularly wishes to thank Carlos and Tere Rodrigues of the Bar Munecas, Covanera, for their unwavering generosity and support.