An ex gliding instructor of mine (Bob Vickers from Lasham if anybody knowshim) told me about a flight in the Rift Valley, where they were in the middle of the valley encounering some mild sink, went the VSI went off the stop in the going down direction (possibly 2000fpm) wings on said glider were very much in the 'U' shape and bob was wondering whether there was a bottom. Very near the valley floor lift returned and they staggered back, i believe it was a Nimbus.
This was a clear blue day. no CB.