[elihu]

Even well-lit caves (or similar environments) can be disorienting. A few years ago I was walking in a sandy-floored lava tube with a flashlight, and the floor was sufficiently uniform and featureless that I didn't have a clear sense of depth perception.

Another time I was flying a radio-controlled glider at the beach on a foggy day. There was a big rock sticking up in the middle of the beach. I thought the glider was closer than the rock, but then suddenly it vanished from sight because it was actually farther away and went behind it. Another flight I thought I was about 20 feet off the ground and then it suddenly struck the sand and slid to a stop.

[watersb]

Flying over water can be dangerous, as it is often impossible to judge distance to the surface, even for experienced pilots,