We kept going until we reached a door. Finally! The corridor was deeper than I thought, like a tunnel. Or the bottom of the sea—the shelves and scrolls becoming the reef and algae that keeps the light from flooding in. A little unsettling. Good thing we were about to leave the place.
I'm not sure whether becoming is the correct choice here. I also thought of turning into but I'm not sure. Maybe I have to rephrase the whole sentence?