Noun
The subterraneous portion of a building, as in amphitheaters, for the service of the games; also, subterranean galleries, as the catacombs.
Source: Webster's dictionaryArena and hypogeum The Colosseum arena, showing the hypogeum now filled with walls. Source: Internet
Detail of the hypogeum The hypogeum was connected by underground tunnels to a number of points outside the Colosseum. Source: Internet
It comprised a wooden floor covered by sand (the Latin word for sand is harena or arena), covering an elaborate underground structure called the hypogeum (literally meaning "underground"). Source: Internet
It has been suggested that the reports either have the location wrong, or that the Colosseum originally featured a wide floodable channel down its central axis (which would later have been replaced by the hypogeum). Source: Internet
Little now remains of the original arena floor, but the hypogeum is still clearly visible. Source: Internet
The depiction of the building itself is generally accurate and it gives a good impression of what the underground hypogeum would have been like. Source: Internet