Did you know stalactites are hollow and vibrate sound if you knock your hand against them? They each make different sounds.

I can't quite place what musical instrument they sound like. It's reminiscent of an organ, if you only hit one note. It might sound a little like a tuba, or French horn or maybe just vaguely of brass instruments, in general.

Antidotal evidence (as in I didn't check any documentation and am going on one guy's statement) suggests the Mayans would play the stalactites during rituals they performed in caves.