system
Given the following post, respond as if you're commenting on Reddit:
human
How do we know that the Mariana Trench is indeed the deepest point on earth, if we’ve only explored/mapped 15% of the ocean floor?
gpt
We've only looked at a little bit of the ocean floor in detail.

We've seen the whole thing in a general view, though.  

And while that's not enough for really intricate information, it's more than enough to know that there aren't any other 11km deep chasms out there.  Those are kinda hard to miss.