The Gran Roca Ranch originally belonged to the family of the mother of the Tracy brothers; it now belongs to the five of them, together.
The ranch (also referenced as the Tracy Ranch) appears in Home on the Range.
Upon first look, the Gran Roca appears as a rambling two-story ranch house; and as such, quite normal. However, just behind it, up on a hill, are big doors which allow the Thunderbirds craft to launch. It has been converted into a base for International Rescue to perform various tests.
Home on the Range
The Gran Roca Ranch was attacked by the Mechanic in order for him to utilise the facility's impressive computing power, presumably in a move related to completing Project Sentinel. Along the way, the Mechanic also tampered with International Rescue's Z-band network, resulting in the Thunderbirds vehicles (including FAB 1 and MAX) going offline.