Ground railroad( secret trail)
dungens
It would still be called a chapel unless if it is just an altar, then it would just be called an altar. A chapel is a holy room reserved for prayer and practising of faith only.
The Sistine Chapel is the best known of the Vatican's chapels.
The Sistine Chapel.
Its Called Holmes Chapel
Underground water is called underground water, because it is "underground" not because it is fresh or salt. You can have underground salt water reservoirs just like you can have fresh water ones.
A basilica is a type of (church) building. A chapel is a part of a church. There is nothing called the basilica chapel.,
This would be the Sistine Chapel located next to St. Peter's Basilica.
Chapel Royal
Bethel
Its Called Holmes Chapel
If its an underground reservoir for water, it could possibly be an Aquifer.