system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: how was it possible for whales to evolve from land mammals to massive ocean mammals?

Did they get a blow hole mutation and start sleeping under water or something? 

What land mammal was it that eventually evolved into a massive ocean mammal?

Why did the whale go back into the water in the first place? Why didnt other mammals do this too?
gpt
Probably from something like seals or walrus, which spend a lot of time under water anyway. Over time, they develop the ability to spend longer and longer periods of time underwater, until they get to a point where they basically never go on land anymore, and they lose the ability to even do so because it never gets used. Voila, whales. And other mammals didn't do it because they didn't need to. Whatever evolved into whales was hunting for food in the water, so being able to stay in the water was advantageous. Other mammals which hunt on land had no need to ever go into the water, so never evolved the ability to do so.