undercroft

noun

1. A cellar or vaulted storage room.

   “The MR [Midland Railway] instead approached its new St Pancras terminus by bridging the canal. This required that the train deck in the Barlow train shed be supported 15ft or so above ground level on more than 800 cast-iron columns (carefully spaced to mirror Burton's beer warehouses), thereby creating the undercroft in which tens of thousands of barrels of beer arriving by rail were stored.”

2. A ground-level car park that occupies the base of a building.