An underground mosque on the island of Djerba in Tunisia.
I got interested in finding out more about underground mosques after seeing a photo of one on the island of Djerba in Tunisia — an island thought to be the same that Homer had in mind when writing about the Lotus Eaters in the Odyssey.
An entry at Wikipedia says that underground mosques are “either erected beneath other buildings or lay freely in the ground with an inconspicuous appearance. The prayer rooms in underground mosques are usually very small, and they also have no minarets. Underground mosques are very rare. [The Djerba mosque] served as a hidden place of prayer for the Ibadis.”
From a different Wikipedia page, we learn that the mosque is near Sedouikech, Tunisia, and dates from the 12th or 13th century. “Surrounded by an olive grove, it opens to the outside by a very steep staircase that leads to the main room; next to it is a large underground tank fed by a well. Another of these underground mosques is located on the Ajim road. Not being used for worship, these mosques can be freely visited.”
An underground mosque in Turkey was built only recently, not because worshippers needed to hide like the Ibadis but because underground worship can feel peaceful.
Menekse Tokyay writes at Alarabiya, “The uniqueness of the Sancaklar mosque is that it departs from standard mosque design in a bid to break architectural taboos and encourage worshippers to focus on the essence of the religious space and on the Islamic faith. …
“Strolling around the mosque’s outdoor area, you will notice a long canopy running along one side where two olive trees and one linden tree are located. From this point, you have to descend natural stone stairs to reach the building.
“The cavernous prayer hall of the mosque is large enough to host more than 650 worshippers, while it aims to isolate believers from the outside world and invite them to delve deeper into their inner world.
“What strikes one about the Sancaklar mosque is that its design is humble and simple, perhaps to deepen worshippers’ relationship with their faith, and with this underground concept, visitors can leave behind all the challenges of the outside world. …
“Sancaklar mosque stands in Istanbul’s suburban Buyukcekmece district and is spread over an area of 1,200 square meters. The architecture combines Islamic and Ottoman designs with a modern touch, seemingly free from mainstream architectural typology.
“In 2013, out of 704 projects from 50 countries, the building won first prize in the World Architecture Festival competition for religious places. In 2015, the project was selected for the Design of the Year award, organized by the London Design Museum and it was also shortlisted among the 40 nominees for the Mies Van der Rohe Award.
“The mosque was designed by Turkish architect Emre Arolat for the Sancaklar Foundation. …
“The only decoration on the walls is the Arabic letter ‘waw’ and verse 41 of Surat al-Ahzab, a chapter in the Quran: ‘O you who have believed, remember Allah with much remembrance.’
“The main space is free of any decorative ornaments unlike many modern mosques built recently in Turkey. Daylight penetrates the prayer hall along the Qibla, or Mecca-facing, wall. …
“ Every time I come here for worship I feel an enormous [sense of] inner peace. It is also a place of meditation for me when praying under daylight infiltrating into the hall,’ Asli Karacan, a youngster living nearby, told Alarabiya.”
Suzanne's Mom's Blog 
No minarets to call for the prayer.
I guess not. The Wikipedia entries were a little rough. I am hoping they were accurate.
When you’re in a Muslim country you really have to get used to those minarets and their muezzins. They call out five times a day, first time around 3 am. It didn’t improve since most mosques replaced their muezzins with loudspeakers that are competing with each other in volume.