The silurian hypothesis is about another smart civilization on earth.
the crust surface under the sea is tectonically recycles more often then land crust
cetaceans as whole returned to the ocean somewhere like 50 million years ago
So there we have it. It’s a bunch of species of futuristic super smart water mammals or whatever they evolved into.
Whales don’t have hands. I feel that octopuses are more likely; they are already aliens.