We are not alone

I think its fair to say that the idea that humanity has not been the only resident of the earth, and it may not currently be the only resident is a staple of horror and conspiracy genres, and certainly of the "secret service" style.

Secret races, antediluvian survivors, and even aliens all seem to fit into this, and most seem to be heavily inspired by Blavatsky.

• What are folks favorite versions of this, the ones that work particularly well for you.
• Can anyone think of non-Blavatsky influenced versions, is that even possible?