Got it in one. They're called Fissures and they've apparently been a problem for a long while now. Something's pulling people through, we're not quite sure what or why.

There's a research station run by some androids who were studying these things. It's a bit of a trip from the city, but it's not that far... they've been letting people dragged through stay there. They've got technology that helps stabilize Fissures--I couldn't tell you the first thing about how it works though--and some of the people staying there go on expeditions to place the stabilizers. A bunch of the people in the city are on one of these expeditions.