What do we call the large burial place under some churches?

The story behind the answer

An underground stone chamber beneath a church, it served as a burial vault and a secure place for holy relics. That makes it distinct from catacombs (vast tunnel cemeteries), a tomb (a single grave or monument), and a chapel (a worship space).

In the Middle Ages, these spaces often supported a raised choir and high altar and became prestigious resting places for clergy and nobility seeking burial ad sanctos—near a saint’s remains. The word comes from Greek kryptē, “hidden.” Many Romanesque and Gothic churches still preserve cool, low-vaulted crypts with sturdy columns guiding pilgrims past reliquaries.