I read it online few years ago. It is a short story about exploring a spacecraft/ship/station that seems to be infinite inside.
It is written like a log, and each entry starts with a measure, like Day 1 "1 km", Day 2 "10 km", Day 3 "100 km" (not sure about dimensions but they grew exponentially).
I found the story at that time while reading about megastructures. Although the construction in the story was huge, it didn't fit the criteria of megastructure because it was infinite.