Back to his roots before he dies, I guess. (Well, the very deepest ones would be "Duel", I suppose, but...)
Polarity Jeff's synopsis of the teaser..(just one guy's speculations)..
https://www.youtube.com/watch?v=dVZNMQfY-d8
Personally I think James Cameron nailed it pretty well in his film "The Abyss".. They aren't from outer space. They've been here for millennia. And they are under our oceans.