During the acid-trip descent, Coop records the important data one can only gather while floating around inside of a black hole. That's not the weird part, though. The weird part is that he ends up inside a tesseract, which is sort of a three-dimensional representation of four-dimensional time and space, and it looks sort of looks like a massive kaleidoscope of book shelves, which sort of looks like the floor of a New Mexico bathroom.
"My god! It's full of ... uh, something!"
It's already arguably ridiculous, but it gets stranger. The tesseract was created by humans in the future who've learned to manipulate time. They know Coop saved humanity by shoving books off his daughter's bookshelf and manipulating the hands on her old watch to play Morse code, so they built him a way to do that. It seems they couldn't build him a chalkboard or a fax machine, because the time in Interstellar is a closed loop -- which we think means it all happens concurrently and the tesseract was always there. So bad news if you were hoping for a movie in which he shoved the wrong book and historically rewrote his daughter to be a Hitler dinosaur.
Oh shit, that would have been a fun Act III.