The Big One: Asteroid Cave or Space Slug Belly site is a special location which must deploy next to a Big One sector, as shown below.

Normally this location is an asteroid cave (planet site). However, when a Space Slug creature is present at the related Big One sector, the location slides next to the Space Slug and becomes a space slug belly (creature site).

When this site is a belly, starfighters may not land or take off if the Space Slug's mouth is 'closed' (as defined by the Space Slug's game text). If the Space Slug is lost, all cards in the belly are also lost and the site then slides next to the Big One, reverting to a cave. Whether the site is a cave or a belly, it is always 'related' to the Big One, and characters, vehicles and creatures may deploy there normally.