Long story short, our sewer main backed up into one of our bathrooms in the basement Saturday night.
We had to have a plumber come yesterday to unclog the sucker. The cause? A damn plastic toy. We didn't see exactly which toy, as it broke and the suction took it away too quickly.
And there were some tree roots as well, so we'll have more clogging in the future if we don't take the tree down. However, 1) we have three large trees in the area of the sewer and because we are on the corner, we don't know if the sewer runs to the side of the house, or the front of the house, so we don't know which tree it is, and 2) we love our huge trees - they give us a lot of shade and they're pretty.