This has been a real bad *$#%ing day - warning pics are not good natured

[Delphinus]

H2S is only a wild guess on my part but here are the main things about it:
1) You can smell it in little as 1ppm air.
2) At 10ppm in a work environment is enough that you should not be exposed to it for more than 10 minutes for every 8 hours. (something like that anyhow)
3) 1000ppm is pretty much instant death.

Hydrogen sulfide (H2S) is a good example of an acutely toxic substance which is immediately lethal at relatively low concentrations. Exposure to a concentration of only 300 PPM (parts per million) for a period of 30 minutes is enough to render a worker unconscious. Exposure to a 1,000 PPM concentration of H2S in air produces rapid paralysis of the respiratory system, cardiac arrest, and death within minutes.
(From http://www.brandtinst.com/biosystems/appnotes/perilsof.htm)

4) We know H2S is produced in anaerobic situations. Perhaps there was some die off in the curing bucket of something and the rot was releasing H2S.

The only other things I can think of:
1) "cuke nuke" - maybe a cucumber was in one of the rocks, and died, releasing whatever neurotoxins they have (some of them have?) ..
2) rapid proliferation of some kind of rot causing the saturated O2 levels of the water to be depleted rapidly?