[growlist]

True! :) I first learnt of them years back on TV - think it was a Channel 4 (UK broadcast channel) documentary, in which they sent a British academic into somewhat hazardous situations. He walked underground along one of these tunnels. Haven't been able to find the documentary since, unfortunately.

[bcraven]

I think Iain Stewart went down one for his "Rise of the Continents" series.