The Underground Mosque, known as Sumur Gumuling”, in Yogyakarta, Indonesia was built upon a circular well in 1750. The building is totally underground, surrounding the well (sumur).
Four stairways above the well meets at the center, forming a small stage and a fifth staircase leads to the upper second floor. These stairways symbolizes the Five Islamic Principles, the highest staircase referring to the pilgrimage to Mecca for Muslims.
The Imam would stand on the small stage formed by the set of stairways, taking advantage of the excellent acoustics of the circular building to deliver his sermon.
The mosque is part of the greater complex of Taman Sari, where underground tunnels link to the Royal Palace, the Water Castle, a pleasure bath complex for the Sultan, his family and concubines and to the South Java Sea. Many of the tunnels are closed for safety reasons.
Much restoration work need to be done in this Complex before it can be officially be listed as an UNESCO Heritage Site.
I have been there and this place is REALLY great and cool.