I thought the Old Ing dive is 2m, deep but 200mm long. (1st one going from Old Ing to Dismal Hill)
No - if I remember correctly, the first one is fairly shallow and about 3 metres long, and leads into a fairly large air-bell. The second starts two metres down and is 60 cm long. It's basically a duck-dive down to an arch, and then straight back up on the other side.
Wow that sounds tricky !! For the one beneath the arch would you use any AIDS ? Weights ? Also does it have a guide line ?
Don't do the Langstroth sumps till you are completely confident in your abilities.We did them the weekend after they were first lined for free-diving and stuffed a diver in first from Langstroth Cave. He reported they were OK so we did the pull-through trip down the Pot.Several years later, we made the mistake of doing the pull-through on a Sunday afternoon as a nice finish to a weekend; the middle air bell (a bit like an upside-down toilet bowl) was completely devoid of breathable air so you just had to keep going. Apparently, this can happen if several parties go through in a weekend and it takes till next week to replenish through aeration or something. I'd always take a mask and a wetsuit hood (brain freeze!) for anything except the short ones.
Regarding: " I'd say the Old Ing deep one almost certainly requires lead, or a heavy slab of rock from the streamway, to get you down there." - I'd suggest caution with this approach. At the downstreanm side you surface in a deep canal which involves swimming some distance. Swimming whilst holding a large rock is far from easy and if you drop it, how will you get back? How will you pull efficiently on the line with both hands whilst holding a gert big slab?Old Ing is a high energy sump in flood, with a greater possibility of abbraided / weakened lines than some other examples suggested above. It's also more challenging than most of the other suggestions. There isn't much margin for error when free diving; I'd advise having the proper equipmend to keep you warm, neutrally buoyant and able to see well underwater.
How dense is rock? It seems to be you need a significant size rock to make a difference.
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