during the times of christian persecutions, the catcombs were a sanctuary were christian gatherings could be safely held, out of the sight of roman authorities.

[link to en.wikipedia.org]

First built in the soft volcanic rock of the Cappadocia region, possibly by the Phrygians in the 8th–7th centuries B.C according to the Turkish Department of Culture,[3] the underground city at Derinkuyu may have been enlarged in the Byzantine era. During the Persian Achaemenid empire the city was used as a refugee settlement. There are references to underground refugee settlements built by the Persian king Yima in the second chapter of the Zoroastrian book Vendidad. Therefore many scholars believe that the city may have been built by the Persians.[4] The city was connected with other underground cities through miles of tunnels.

Some artifacts discovered in these underground settlements belong to the Middle Byzantine Period, between the 5th and the 10th centuries A.D. It is speculated that the number of underground settlements, generally used for taking refuge and for religious purposes, increased during this era.

[link to io9.com]
Humans like to build cities that reach for the sky, but sometimes they also build downward. Whether for protection, or just because they can, we've created many cities beneath the Earth's crust - or dug deep into the rocky faces of mountains. Here are ten of the most awesome.