Kayaking around Vang Vieng, in Laos. We got out of the kayaks several times to visit various different sites, including two very large caves. At the first one we had to wait quite a while at the entrance as a bunch of locals climbed out in pairs, each pair carrying a long piece of lumber. This was extremely confusing until we finally got out of our guide that we were in fact going right through the hill and out the other side. The second cave was also a very large walk-through, and the exit was quite a scramble: right outside the entrance a bunch of monks were waiting - we don't know why.