Interesting layers for Google Earth

Alright folks, I’ve already looked at .kmz and .kml layers of medieval castles and fortresses around the world, WWII British defence sites, and ancient sites within Rome. What other cool layers do you all know of? And where can I find them?

I forgot to post the links to those layers I mentioned. It’s only fair that I should share at least one if I’m going to take as well.Here Is a link to a google earth community where you can download a layer showing forts around the world.