I really doubt this trip can compete with the trip to Murphys in Killarney, Ireland next month, but I'll be open minded about it.
Speaking of catacombs, the catacombs under Paris are part of an extensive system of tunnels that had been used as stone quarries for centuries. In the late 1700s, cemeteries became so crowded and unsanitary that the government condemned many of the inner city graveyards, dug up the graves and moved the bones to a section of the quarries, now known as the catacombs. The remains of about 6 million people are there.
Beyond the catacombs, the underground tunnels cover vast areas. Only certain parts are open to visitors, although some people, called cataphiles, explore the underground world on their own, illegally. More on that later.
A wall of human bones in the catacombs.