Our friends Pete and John, who are pro plumbers, got to work:
Here's what they found: A "Little Shop of Horrors" bittersweet vine had infiltrated the main sewer line.
The culprit - no wonder this vine stayed green all winter.
Adding a PVC line w/cleanout. By the way, our culprit's little brother is climbing the house on the right. Guess we better get him removed, too.
By the way, John found a newspaper fragment in the crawlspace about the U.S. about to go to war with Japan.