Archaeologists working for the Israel Antiquities Authority (IAA) recently excavated a unique ritual bath (mikveh) in the Kiryat Menachem neighborhood of Jerusalem (southwest of the Old City). Three separate collecting tanks (otzar) were carved into the rock in order to collect as much rainwater as possible.

The IAA press release quotes Benyamin Storchan, director of the excavation:

Numerous ritual baths have been excavated in Jerusalem in recent years, but the water supply system that we exposed in this excavation is unique and unusual. The ritual bath consists of an underground chamber entered by way of steps. The miqwe received the rainwater from three collecting basins (otzar) that were hewn on the roof of the bath, and the pure water was conveyed inside the chamber through channels.
The ritual baths known until now usually consist of a closed cavity that was supplied with rainwater conveyed from a small rock-cut pool located nearby. The complex that was exposed at this time is a more sophisticated and intricate system. The bath was apparently associated with a settlement that was situated there in the Second Temple period. Presumably, due to the rainfall regime and arid conditions of the region, the inhabitants sought special techniques that would make it possible to store every drop of water.
It is interesting to note that the bath conforms to all of the laws of kashrut, like collecting the water in it naturally without human contact, and ensuring that the water does not seep into the earth which is why the bath was treated with a special kind of plaster.

The full press release is here, and three high-res photos are available in a zip file. The story is also reported by the Jerusalem Post and Arutz-7.