The passage tunnel in the Desert Sector is a building which makes it possible to travel to another sector.
It can be reached by meeting the passage tunnel requirements of the Volcano Sector's passage tunnel, by winning a fight in the Volcano Sector's arena.
The access to the remaining sectors is automatically granted to any mechmind that reaches this passage tunnel, without meeting any requirements anymore.
Trivia
Sometimes on very rare occasion passage tunnel might darn an Unknown mechanoid. It has noted to be happening in Desert the most. Such phenomen has been noted to take a place in version 1.0.3.1, but there's a chance that it might also happen with other versions below and above that number.