Tuesday, January 11, 2011

Deep Thud

kw: wild caves, spelunking, potholing

Talking to someone about climbing and rappelling reminded me of a memory from forty years ago. Though I have done quite a lot of spelunking, and participated in a couple of surveys in wild caves, I've never rappelled. But I have friends who've dropped into some of the deepest holes on Earth.

This shows one of them; you see here the 50x60m opening of Las Golondrinas (Cave of Swallows) in central Mexico. You can see the scale by the outbuildings of several farms in this image. The floor of the pit is 370m below the upper lip.

In 1971 I attended a show-and-tell session at a NSS (National Speleological Society) meeting, where a group of "potholers" shared their experiences of rappelling into this and other deep-shaft caves in the central Mexican mountains. The sides of the pit contain many holes; the entire mountain range is cavernous limestone. Thousands of birds nest there, mostly swifts, swallows and parrots (conures). The presenters showed many pictures at this meeting, but the most memorable item was a half-minute-long tape.

After spending a couple of nights in the cave, these folks decided to do a rock drop and record the sound. Leaving their last member at the bottom with a tape recorder, they selected a 20 kg stone and dropped it off the upper lip of the pit. The recorder was left running near where the stone would most likely hit, and the fellow moved away. Early in the recording we could hear the last walkie-talkie messages, then a few seconds of bird noises that get quiet as a thrumming, whooshing noise increases. In a vacuum, the fall would have been nine seconds, and the stone's velocity would have approached 300 m/s. The fall actually lasted almost fifteen seconds, and the bird noises entirely stopped during that time. Then there is a huge THUD!, and it sounds like every parrot in Mexico screams!!

I have searched for an online copy of the recording, without result. I wonder if the tape still exists (or an mp3!).

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