In the Gears of War universe, the Hollow is a vast network of underground tunnels that span all the way through the crust of the planet Sera. This subterranean realm is home to the Locust Horde and many other creatures and is divided between the Inner Hollows and Outer Hollows. Considering how far and wide this network stretches, the Hollow is still mostly uncharted by humanity.
The Outer Hollows have been partially mapped by humans, though much of it is still uncharted territory. Humanity has at least been aware of the Outer Hollows' existence for a considerable length of time before the Locust War since they have been using them as dumping grounds for centuries. Ironically, the Locust scavenged much of the waste material humanity had discarded and used it to build up their war machine against the humans.
The Inner Hollows are the cradle of Locust civilization where their capital city of Nexus is located. Containing numerous natural lakes as well as massive reservoirs of Imulsion, most of the Hollow's indigenous life-forms can be found here. Humanity has never explored these depths and the only time humans have ever travelled this deep was during Operation: Hollow Storm, when the Coalition of Ordered Governments attempted to defeat the Locust once and for all by sinking their own capital city of Jacinto, flooding the Hollow and wiping out the Locust. As such, the Inner Hollows are now flooded and abandoned.