A "sewer line belly" is what happens when your main drain pipe sinks because of bad soil. Most of the time, this is because of a bad job of installing the sewer line. Drain bellies can happen on their own when tree roots grow into the drain or when the weather changes in a big way. When something sags, it makes a trough. In the trough, grease, sludge, trash, and other things end up. If sewer line bellies aren't fixed, they can cause erosion, clogs, and sewer back-ups.