Caves
The signature of a visitor from 1897. They used pencil thinking that it would be erasable later on. A volunteer in the 1930s went back with a huge eraser to try to clean it all up, and was unable. What happened was that the formation is made of many clear layers of crystal, and between the time that the people had written on them, and they were to be erased, a few layers had been deposited on top, preventing their erasure.