Do you think, atlantis was real in past?

Zero evidence.

I think maybe people witnessed aquatic mammals choosing to exist on land rather than water. Which led them to believe there was a land beneath the water.

Nmh.

I there are similar legends in other countries too.

Too much of a coincidence.

Nah.

No!

I don't know the myth's origin but wasn't it largely talked up by novelists anyway, or even invented by one?

Yep