One of the strongest facts of science: Whales evolved from land animals.

Whales evolved from land animals. The fossil record is complete. And all this is confirmed by DNA sequencing.

From page 51 of Why Evolution is True by Jerry Coyne:

"Finally, at 40 million years ago, we find the fossils Basilosaurus and Dorudon--clearly fully aquatic mammals, with short necks and blowholes atop the skull. They could not have spent any time on land, for their pelvis and hindlimbs were reduced (the fifty-foot Dorudon had legs only two feet long) and were unconnected to the rest of the skeleton."