Hi Pete,
It's been such a long time since diving in the sea, I'd forgotten about that. If I remember, a diver is 2.5 to 3% deeper in fresh water than the gauge shows, unless calibrated for fresh water? So the above mentioned dive is about 1m deeper, 39.5m. Sooo glad I didn't know that at the time. I'd seen a fairly snug rift leading off from the bottom and descended in absolute zero vis before it felt like it pinched in, so turned back. Once in better vis, everything seemed a lot darker. On the deco, realised a double dive torch had come off my helmet in the low passage.
The following week, I went back. Just before the mud rolled in near the dropped reel, saw the torches, and even better, a larger passage leading off at the bottom. Being young and foolish, I picked up the reel and rushed into it without putting on a belay. Within 7/8 m, it closed down, but above me was a big, crystal clear void. I started to swim up into this, with the intention of rejoining the line around the base of the chamber. But I quickly realised I might not be able to find the line, and would have to return along the low passage with even less air. Dropping the reel in an absolute blackout, within a few m's the line went into a 3 cm slot, and it felt tight all around. Thinking 'sod this', I went back to the reel intending to exit via the big clear space, hopefully. However, the blackness was now all prevailing and all I could find was solid rock. At this point, I felt close to panic, and my breathing rate went right up. Decided it would be better to find the way out, so returned to the 3 cm slot to try and find that passage. I have never felt such a surge of relief as when I put my hand on the last lead block belay. And despite mild hypothermia from 30mins of deco due to tables, and a wetsuit with too many holes, I just couldn't stop smiling.
I've included the above story to dispel the myth that Prid is a 'safe' dive. Staying on the mainline, with a max depth of 24m and clean overhead rock it's fairly straightforward. It was afterall my first cave dive. But if you leave the rope line and venture more into the chamber, the silt rains down on you from above, the floor is deep incredibly fine silt, and the water is deep enough to narc you. I almost panicked when I re picked up the reel and could only find rock. Luckily, I was able to focus on what had to be done, and survived. So, a great dive, yes, often sensational vis. But a 'safe' dive, I think not.
Gus, I've often wondered about the chamber and how it formed. There appear to be very few boulders on the floor. Though the silt could be concealing much, like the big river passage that leads out to the sea at Berry Head!