For those who do not know Leviathans are the giant green dinosaur creatures in Titanfall. They come from a planet of the same name, in the Badlands System. On this planet we have the maps Swampland, Boneyard, Rise, and possibly runoff but I can't confirm that one until I get into a private match. And they are featured on all 3 maps. In Swampland there is the skeleton of a Leviathan in the back of the map, in Boneyard well they're everywhere, bones and living. And in Rise, on the surface the map Boneyard and a giant leviathan skeleton are visible in the distance. We know these are the same planet and that makes sense for them to be there, but how did they end up on Airbase Sierra? Airbase Sierra is located on one of Demeter's Moons, Demeter is the closest planet to the core system and Earth. How did these giant unintelligent creatures get so far from home?

TL;DR Something big either happened or plot holes.