Open Sesame

Despite the padlock being securely in place, the gate to the ancient basilica was open...okay, so I know that this is possible because the bolt part of the lock doesn't go far enough into the hole to stop you from pulling it open, but who told the tourists?

The reason for the gate is the dark rectangle just above the centre of the picture. It is a very old metal grille covering a very deep hole (the catacombs I presume) with no safe way down or up!