In 1900, so the story goes, a donkey fell through a hole in the ground and disappeared. This was in Alexandria, Egypt, where such events are far from rare, so the donkey's owner went to the museum and asked if someone could come check the site quickly so he could resume work.
His work was never resumed. The donkey, if there was a donkey, had fallen into a great shaft 18 feet wide descending four stories into the earth. A spiral staircase runs around the outside of the shaft, separated from it by a wall.
The staircase takes you down to this spectacular chamber with its weird mix of classical and Egyptian imagery. This seems to have been built in the 100s AD.
Around the central chamber are several smaller rooms, decorated with themes that include the mummification of Osiris (above) and the abduction of Persephone.
The original route to the catacombs themselves was blocked by rubble, so today you follow this tunnel dug by medieval grave robbers.
The catacombs themselves are plain, their paintings long faded. That is, the ones we can reach. There is another, flooded level beneath this one, but if one of those Archaeology as Adventure shows has ever sent people diving in to investigate I can't find a record of it.