Lone Eagle in the Nest: preparing for a solo dive in the Eagle's Nest sinkhole.

1. Put some helium in the mix. Sheck Exley dived this sinkhole to 300+ feet on air, and talking to people it is clear that some still feel air is the right gas to use in the Nest; but Sheck didn't have helium available at that time and we do. I feel strongly it is wrong to even think about using air this deep. Put some helium in and keep the PO2 down below 1.6. That was my first decision. (A good friend was worried about me diving alone in the Nest, and told me a story of how he and his buddy had been diving it on air at 240' when his buddy went in to this "asleep with his eyes open" state which is one of the ways deep air can kill you. However, he towed the buddy towards safety and he suddenly came to around 140' deep. But I'm afraid that doesn't put me off diving solo; it puts me off deep air.)
2. Have a look on air, since you can do a little bit that way and check it out. Air is cheap, helium expensive. So I had a look at what you can do on air before doing the main dive on Trimix. If the visibility had been bad, or if I had committed the terrible crime of not liking the cave, I would not have done the main dive (but you MUST like the Eagles Nest..... well, I reserve the right to disagree.... but in fact, I did like it very much). I limited myself to 190 feet on the air dive (PO2=1.42; experience suggested I wouldn't feel much nitrogen narcosis at that depth either, and in the event I felt none at all), seeing a little bit of the upstream and a little bit of the downstream from the bottom of the entrance shaft.
3. Choice of Bottom Mix and Depth. I decided to go for 14/33 trimix, which is a complicated way of saying that I filled my cylinders 1/3 full of Helium and topped them off with 2/3 full of air. I had been informed that I would probably get to not deeper than 250 feet, at which point I would have both Nitrogen and Oxygen at partial pressures same as for an air dive to 156 feet, well within my confidence zone (PO2=1.20; PN2=4.52). Somewhat cheaper than the 15/50 Trimix which some use; and OK to 250 feet.
4. Take Plenty of Travel Mix. I used a deco program on a PC to plan my dive, and it predicted that even with the most conservative of 3 deco schedules, I could get by with 80 cubic feet of 32% Nitrox travel mix. I nonetheless took 2 Alu 80 cylinders (160 cubic feet) of travel mix on a just-in-case basis.
5. VPM or Buhlmann? On the subject of deco planning, we currently have this choice between the tried-and-tested Buhlmann algorithm and the mathematically more satisfying but less well tested VPM decompression algorithm for the mathematical calculations behind our deco planning (there are a few more variants but the key point is VPM gives you deeper stops but shorter shallow stops). Now I had already experimented with deep stops on shallower dives (e.g. instead of having a safety stop at 10 feet, if I had some compulsory deco at 10 feet anyway I would do safety stops of 1 minute at 30 feet and 2 minutes at 20 feet, plus whatever I needed to do at 10. If I had compulsory deco at 20, I would do the safety stops at 40 and 30). I had already found that this eliminated excessive tiredness at the end of dives, unless I got cold during the dive in which case the tiredness would return. So I was already a fan of deep stops, and decided this would be a good dive to use VPM on. But I decided to be very conservative in my use of VPM since it was my first time with VPM deco.
6. How conservative? I made three deco schedules. All were based on VPM with 10% safety margin, but the estimated dive profiles were different in the following way: all three began with a swift descent to 190' (the depth I'd already been to) and ended with an ascent from 190' according to the VPM calculations; the first profile involved, in the middle of the dive, a slow descent over a period of 10 minutes from 190' to 250', followed by an equally slow ascent back to 190'; the second profile involved a slow descent over a period of 15 minutes from 190' to 250', followed by an equally slow ascent back to 190'; and the most conservative involved a slow descent over a period of 20 minutes from 190' to 250', followed by an equally slow ascent back to 190'. In the event I took between 10 and 15 minutes each way during the dive, and went to about 235 of the available 250 feet, but I used the most conservative deco schedule as for 20 minutes each way to 250 feet. OK it's overcautious, but this was my first time with VPM deco.
7. Gear configuration. I used the same sidemount configuration I had used all through the trip. It is very helpful because (a) the two bottom mix cylinders give full 100% redundancy and (b) if a regulator freeflows at depth, you can easily reach the tank valve and control its flow on-off breath by breath. The deeper you go, the better this feature looks. It means that every regulator you have can simultaneously decide to freeflow, but you will still have access to all gas supplies via all regs. And if an A-clamp O-ring decides to extrude, you can see it to fix it without de-kitting. And if a hose decides to leak at the valve end, you can immediately see what's causing the problem and perhaps fix it. Etc., etc.
8. Buoyancy. As always with a dive in a place where crawling out with negative buoyancy would be problematic, I had redundant buoyancy. Method 1 of achieving buoyancy was to inflate my drysuit; method 2 was to inflate the wing which I had strapped across my chest (even when sidemounting, I prefer a wing to a conventional BC). I arranged that the drysuit be inflated with travel mix and the wing with bottom mix, because you don't want helium in your drysuit, its superior thermal conductivity will give you hypothermia. That meant that I didn't inflate the drysuit during the bottom part of the dive, because I decoupled my travel mix at the bottom of the entrance shaft (120'). This was OK to 235 feet, but I'd have to arrange a special feed for the drysuit if going much deeper - it was beginning to get tight by the deepest part of the dive! A small amount of water leaked into the drysuit while its feed was open to the water when the travel mix was disconnected, but I've been there before, and my damp shirt was quickly dried afterwards by the Florida sunshine.
9. Gentle return to travel mix. Probably doesn't matter for medium depth techdiving like this, but it can be dangerous to return too quickly from a bottom mix with helium in it to a nitrox travel mix, it can lead to a nasty fast-tissue DCI, the "vestibular hit". (This is what is thought to have killed British cavediver Rob Parker, who changed instantly from Trimix to air after a dive to 100msw (330 fsw), changing rather deep because he had run short of bottom mix. He was swiftly knocked unconscious by this change of gases, and the buddy was unable to prevent him sinking back deep and drowning). So I tweaked the VPM deco plan. Instead of changing instantly from bottom mix to travel mix at the 1 minute 120 foot stop, I alternately took 1 breath of travel mix to 2 breaths bottom mix at that stop, then 2 breaths of travel mix to 1 breath bottom mix at the 115 foot stop (my stops were graded at 5 foot intervals), then pure travel mix from 110 feet onwards. Probably another example of overcaution, because I wasn't going as deep as 100m and very definitely did not plan to run short of bottom mix, in fact ending with almost exactly 1/3 of my bottom mix unspent, as one should when the dive is without crisis and the thirds air supply rule is followed.
10. No pure O2 at the 20 foot stop. I don't do 20' stops on pure O2 because the PPO2 is minisculely above 1.6. So I increased its length, did it on travel mix (32%) and changed to pure O2 at the 15 foot stop. Probably another example of overcaution.
11. Large multi-page Dive-Rite wrist slate with deco schedules clearly written on it. 'Nuff said.

Having completed the dive and deco successfully, I was just heading for the surface when I dropped my O2 bottle in the swamp vegetation of the sinkhole. It took me half an hour to find it again!! Those weeds surely get their revenge if you drop a tank in them!! Next time, let's take the extra 5 seconds and clip the O2 bottle where it belongs shall we, Prof. Read??

Concerning the reputation of the site, I think I can see how it is so dangerous; it flatters to deceive! It has in common with places like Little River that it seems a really friendly cave (good visibility; clean light coloured limestone; plenty of space; not too much silt at the start of the cave), but of course the dangers are nonetheless there. A story to be continued.