On a visit to the Capuchin Convent, the group is led into a vault decorated with the bones of ancient deceased monks. What is it about this vault that so fascinates Twain?

The simple grotesqueness of the vault sparks Twain's interest. Who would choose to decorate their holy space with the bones of dead occupants? It is simply beyond comprehension. There are thousands of bones dating back to thousands of years, some with romantic stories attached. Twain is fascinated by the personal history as well.