Question: What did the underground passage connect to the Colosseum?
About the article "Colosseum", section "Supporting buildings".

Candidate passages:
The Bourbon Tunnel or Tunnel Borbonico is an ancient underground passage, constructed for military purposes to connect the Royal Palace to military barracks in Naples, Italy. 
The iconic status of the Colosseum has led it to be featured in numerous films and other items of popular culture:  In the 1953 film "Roman Holiday", the Colosseum famously serves as the backdrop for several scenes.
In the 1954 film "Demetrius and the Gladiators", the Emperor Caligula anachronistically sentences the Christian Demetrius to fight in the Colosseum.
The conclusion of the 1957 film "20 Million Miles to Earth" takes place at the Colosseum.
The Bob Dylan song "When I Paint My Masterpiece," first recorded in 1971 by The Band and later appearing on the album "Bob Dylan's Greatest Hits Vol.
II", mentions both the "Spanish Stairs" (the Spanish Steps) and the Colosseum.
In the 1972 film "Way of the Dragon", Bruce Lee fought Chuck Norris in the Colosseum.
In Ridley Scott's 2000 film "Gladiator", the Colosseum was re-created via computer-generated imagery (CGI) to "restore" it to the glory of its heyday in the 2nd century.
The depiction of the building itself is generally accurate and it gives a good impression of what the underground "hypogeum" would have been like.
In the 2003 movie "The Core", the Colosseum is destroyed by huge bolts of lightning.
The Colosseum was featured as one of the locations of Vitaly's travelling circus in the animated film (2012).
The Colosseum features prominently in the video game "".  
The Colosseum and its activities supported a substantial industry in the area.
In addition to the amphitheatre itself, many other buildings nearby were linked to the games.
Immediately to the east is the remains of the "Ludus Magnus", a training school for gladiators.
This was connected to the Colosseum by an underground passage, to allow easy access for the gladiators.
The "Ludus Magnus" had its own miniature training arena, which was itself a popular attraction for Roman spectators.
Other training schools were in the same area, including the "Ludus Matutinus" (Morning School), where fighters of animals were trained, plus the Dacian and Gallic Schools.
Also nearby were the "Armamentarium", comprising an armory to store weapons; the "Summum Choragium", where machinery was stored; the "Sanitarium", which had facilities to treat wounded gladiators; and the "Spoliarium", where bodies of dead gladiators were stripped of their armor and disposed of.
Around the perimeter of the Colosseum, at a distance of 18 m (59 ft) from the perimeter, was a series of tall stone posts, with five remaining on the eastern side.
Various explanations have been advanced for their presence; they may have been a religious boundary, or an outer boundary for ticket checks, or an anchor for the "velarium" or awning.
Right next to the Colosseum is also the Arch of Constantine.
The Colosseum is an elliptical amphitheater in Rome, Italy.
Colosseum or coliseum may also refer to: 
"The Collectors' Colosseum" is probably the first compilation album by Colosseum, released in England in 1971.