Oxford University Cave Club

Diving the Trench Series, Pozu del Xitu, -362m

18th July 2014

Paul Mackrill

I used to just take a small Premiere Carbide, woolly underwear, helmet and a chest band tied from a bit of climbing tape for a life line point. It fitted into a small basic canvas rucksack. 5 of us would trail off in a small car to the Yorkshire Dales from Lancashire. Now I have filled the car with a 300 bar diving compressor, 5 cylinders, 1500 lumen lighting systems, full SRT kit, 7mm wetsuit, dry caving suit, kit bags, etc. The car is full and I am travelling on my own!

My objectives are twofold. There is a sump at the bottom of the trenches series which has been a conundrum for all the years since its discovery during the original exploration at the end of the 70’s. The original end of Xitu had dropped down a series of pitches to a sump and the water headed off to an unknown destination. The second objective was the sump lying at the bottom of C4 and the survey showed it “inches” from 2/7.

Unfortunately only the rigging of Xitu was completed in time for an exploration of the underwater world. C4 was rigged to the bottom of the Monster and is left for next year’s objectives.

So on the 15th July, with a small team [Rowan, Nathan, Rosa and myself], we set off with the objective of either rigging to the sump or finding a sump bypass.When I arrived the trenches had been rigged to the bottom of the main pitches and through a traverse to the final cross rifts [just after, I assume, Chopper pitch]. I had the choice to follow the original route, down Grey pitch, which led into what was described as crawling through loose rocks and mud OR attempt to bypass the sump via the upper rift only visit by John Singleton on the original explorations in 1980 [?].

This was a good choice as the rift, after a brief narrow section gave way to a high traverse. As the traverse opened out I chose to rig a wide rift down some 20/25m to an area of loose boulders with a stream running in a narrow slot under the boulders below, which would have been the original route to the sump. I traversed back upstream and came out opposite the original route on the ledge 5m up from the bottom of Grey pitch but I couldn’t climb down to the rift to the original route to the sump. I had arrived here via the “parallel boulder-filled rift which was not followed as it seemed to choke very quickly” described in Proc 9. Back at our arrival point Nathan had found a way on via a climb up into the boulders and past a couple of windows down into the rift to the stream. A further short sandy crawl led to a drop down an ancient mud-filled muddy vadose streamway which led around a corner to 6m drop to the long sump pool.

The sump pool looked cloudy with clean rock walls rising above the water for about 2m, suggesting underwater might be cleaner. The new approach gave a good kitting up area and I felt that an exploration dive was on.

I got a small group to drag all the dive kit up with me to Ario, then on the 18th July we set off as a small and motivated team to attempt the sump [Eabha, Jack, Nathanial and I].
I took in a pair of 4 litre cylinders and stripped back the weights to just 1kg as I’d be diving in SRT gear [additional weight]. I also feared the sump would be deep and didn’t want to enter the negative buoyancy territory; and I’d compensate with a bit of finning to get me down. As it was I was almost in perfect neutral buoyancy in the sump.

The cylinders were protected with Karrimat and the valve heads with used Mornflake tins, which survived the carry through climax rift and all the obstacles beyond to arrive at the kitting area in one piece. I laid the kit out on the empty bags to avoid getting mud in the valves and joints, and I changed on Eabha’s survival tent.

The entry to the sump had me on totally new territory. I had to negotiate a 6m vertical and muddy walled pitch into a sump of unknown depth. We had forgotten a spare line to lower the ancillary kit such as fins and dive real; so I elected to abseil with the full kit in place, so when I hit the water I would have everything in place. I prefilled my scoff buoyancy bag to be sure I didn’t sink out of sight on entry as I had no platform to pre-test all the kit and be sure of my trim. The cup of hot tea I had before I set off was most welcome.

I hit the water and was almost relieved at its cold 6°C grip as I was pretty warm after all the kitting and entry antics. However I was now standing on a shelf of sandy mud which was some 1 metre below the water level.

I had wound the dive reel with 100m of line, but the conditions meant that it wasn’t easy to lay the line with ease from a bobbin; so I shed 20m by getting the team to pull in some line until I had a tidy line reel.

I dived the length of the pool looking at the floor for a clear way on. Under water it was narrower than I expected and my side mounted tanks scraped both walls. I choose not to drop down immediately until I had secured the line as, with the line coming in at an angle, I feared a line trap lower down as the sump could have narrowed. Near the far end I found a flake, put in a snoopy and dropped down into clearer water. Another flake and snoopy and I was moving forward under the far end of the sump and into the unknown. But the wall dropped down in front of me and formed a curved alcove. I let myself sink hoping the way was underneath but the vis below was zero and my hand hit mud. I felt from side to side and the mud lay evenly across the bottom.

So I surfaced and made two more direct descents at different points along the rift. I always reached a zone of zero visibility and my hand searching in the gloom only found sandy mud from wall to wall.

I finally surfaced and called it a day. Any way on would require digging and the fill seemed to be pretty even at about -6m from the sump surface. I now had to get myself out and I took off my fins and attached them to my hand jammer. I then had the joy of SRT climbing the 6m pitch to the ledge fully kitted, which surprisingly straightforward. It was a matter of dekitting, redressing and eating before the 362m climb back to the surface giving an 11 hour trip.

So what have we got? In my opinion the sump is a mud filled rift with the water seeping through the sandy mud to a probable rift. It could escape the rift in either direction. The water backs up to at least 10m above the sump in flood and there is mud on all the approaches to the sump. Although the pitches are huge leading down to the bottom of the first canyon, the connection through the steep vadose passage of the way to the Chopper shows the more immature nature of this passage. Most of the water in Xitu went into the Teresa series, the trenches only being a relatively immature sink created much more recently.

Later attempts to bypass the sump at a higher level [along the passage followed by John Singleton] by Eabha and Rowan showed that the passage split into two parts, and both closed in and dropped back to the final sump. As yet no way on in this area has been found. Earlier dye tests have been made but I do not yet know of the dates and their results. For me this was a very useful exercise in conditioning and carrying dive equipment for deep caving. Thanks to all that helped make this possible.