The photography group I belong to had a private tour of Crystal Caves, a limestone cave located in Sequoia National Park. Their regular tours are pretty crowded, and they do not allow tripods. It was wonderful to be able to spend two hours inside the cave with just our group of 12 and the rangers who accompanied us, and they did allow tripods. The rangers were very accommodating and willing to do anything they could to help us get good shots. This is probably the most challenging conditions I have ever worked in. The cave is, of course, dark, so focusing is not easy. You're carrying your equipment while navigating uneven, wet surfaces and narrow passages in the dark. And it's sometimes drippy, so you have to take care to keep your equipment dry. I hope we will be able to do this again, because I think that I learned some things this time that would enable me to get more keepers next time. But I did get these few that I think turned out fairly well.