Apse and Underbelly

Apse noun - A domed or vaulted recess or projection on a building especially the east end of a church; usually contains the altar.

Underbelly and apse are semantically related. In some cases you can use "Underbelly" instead a noun "Apse".

Underbelly noun - A social sphere that exists outside of the mainstream.

Apse and underbelly are semantically related. Sometimes you can use "Apse" instead a noun "Underbelly".