The Mire

"All places are alike, and every earth is fit for burial."
Terra Firma is a massive underground "colony of dead" found in a massive Equestrian-controlled catacomb.

Equestrians that can afford the expense give their dead family members the honor of being buried with their ancestors in Terra Firma a few hundred miles into the wilderness to the north, near the Junia mountain range. This is an ancient tradition that Equestria has held close and dear for millenia (despite the strain it places on the country!)

Terra Firma is maintained by a civilian population that lives in the underground city there. Because of the holiness of the location, there are also several thousand soldiers from both the White Army and the Night Corps stationed there. The mass grave is protected by a massive boulder that takes several hundred unicorns to move out of the way of the cave entrance.