The Moon’s not artificial, but could very well have voids within either larger and filled with water (which would reduce lunar mass) or (much) smaller and formerly filled with water (analogous to a cellulose sponge). Seems to me the Sagan citation refers to a place where Sagan discusses the possibility of the artificiality of the two major moons of Mars, Phobos and Deimos.
http://www.freerepublic.com/focus/chat/1234919/posts?page=4#4
What do I know.
The ringing ‘like a bell’ is an interesting repeated thing, evidently.
Doesn’t change my theology one way or the other.
Seemed like an interesting list of stuff.