mithraeum, n.All

The Mithraeum (n.) under ⁠​‌​​​‌​​​‌​‌‌​​​​‌​‌​​‌‌​‌​​‌​​​​​‌‌​​‌​​​‌‌​‌‌​⁠San Clemente in Rome is still full of water; pump it out and it fills again from a spring that has never been found.