Here's a great clip from Mac Tonnies, being interviewed by Paul Kimball for the upcoming Best Evidence: Top 10 UFO Cases, on the cryptoterrestrial hypothesis. It's very well explained by Mac, and really quite plausible if you take into account many cases and take into account folk lore and mythology.

While there are, of course, many difficult questions that need to be explored, I think there are in any explanation of the phenomenon. It does seem to answer many questions that other explanations don't, so I think it makes for a really wonderful thought experiment. His upcoming book the subject is going to be an excellent read, I believe.