"I wondered what a crypt was like"
Crypt of the Church of St John the Baptist, Berkswell, W. Midlands
GNU Free Documentation LicenseCrypt of the Church of St John the Baptist, Berkswell, W. Midlands - Credit: Oosoom

A crypt is an underground room or vault, located beneath a church or cathedral, where the bodies of the deceased are stored. They are almost invariably made of stone, and often have vaulted ceilings.  

Information about the Crypt at St. Pancras Parish Church in London notes that it was used for 'coffin burials from 1822, when the Church was opened, to 1854, when the crypts of all London churches were closed to burials'.

It is not clear whether the burial of a coffin in the church crypt (as seems to occur in the case of Rebecca's coffin) would have been usual in the period in which Rebecca is set.