Palazzo della Cancelleria is a Renaissance palace in Campo de’ Fiori currently hosting a temporary exhibition of Leonardo Da Vinci’s machines (which actually has been going on ever since I can remember!). But the quirky thing about the Palazzo della Cancelleria is that underneath it (and therefore underneath one of the most popular squares in the city centre) there is a real lake submerging a Roman tomb, which you can see once you get a ticket for the current exhibition.
The lake was discovered during the 1930s during some excavation works in the basement. It originated from the clog of the narrow canal Euripus, which ran underneath Campo Marzio as far as the Tevere river. The colour of the water is mesmerizing, but in my opinion the best feature of this spot is simply being there, hidden underneath one of the busiest squares in Rome. It creates such a weird, parallel dimension to step into!