So, as an entertaining movie it's very nice, but when we look for coherence there are stuff they just didn't care providing an explanation.

So something created the time trap as a way to protect the fountain. But what exactly is it, and who are they? Ate they the guardians or the guardians are somebody else? Would they be aliens that for any reason made that and left? If they were able to create a time trap, would it be fair to consider they're able to time travel too, and are humans from waaaaay the future? But if so, why'd they place the fountain and the trap there? Maybe somehow they need it there to be able to travel in time and not want ppl thru time messing with it?

If american ppl knew about it to the point of calling it fountain of yought, then shouldn't somebody have entered and left it?

Hopper used a horizontal cave which is easy to enter and leave. So it seems implausible that nobody else found it, specially the idiocracy of the cave men, even them would be able to find it given the amount of time they were there.

And it's shown that life on Earth was destroyed and they were going to Mars. It's another idiocracy, because hardly somebody unable to sustain life on Earth would be able to on Mars. And why'd they have all that trouble to save ppl on the cave, even more staying there outside waiting eaons for somebody to climb? And how did they know they were there?

The man on the suit went directly to the small fountain, so somehow he knew it existed. The knownledge should have died with Hopper, and if the guy knew enough to go to the small fountain he'd also know enough to not go there without a rescue plan.