[Driftwood]

<font color='#0000FF'>Yup, this came up at the weekend on our Advanced Nitrox course. The talk was (as already pointed out by Digger) that the actual depth was different to the stated due to the tidal flow preventing the line from dropping vertically. Also, there was no exertion involved (ie sink down and inflate/dump up) as anything more than a small movement would have triggered just enough CO2 that at that depth it would cause mass panic, narcosis and death.

If you wonder how Tanya Streeter et al do it on a breath hold, the second part of the mammalian reflex stops their heads imploding as the lungs and sinus fill with tissue fluid (a.k.a plasma) which is incompressible.