Great Catastrophe

The Great Catastrophe in Blood Blocade Battlefront is the premise indicator behind the entire pollen of Kekkei Sensen. Formerly in the synopsis the show states the city is based in New York City 3 years ago. From Episode 8, they explain that there's a "Hole." and somehow this event could have ended the entire world except for "Casters."

History
Somewhere between 20 years prior to the show the great catastrophe blocked the city off from the rest of the world through "The Great Barrier."

Elaboration
This great barrier somehow is not entirely explained in the anime. The show in Episode 6 indicates major food chains somehow can supply their restaurants through the great barrier.

In episode 3 the show indicates there's passage ways threw unnatural means that we are to assume that gateways allow indirect travel through dimensions as there's a road leading to the Ponsei game.