Unfortunately, this is no urban legend.
Here’s a fascinating (and horrifying!) video from the Seattle Channel (public access television) about the explosion in Seattle’s rat population this year.
Fox President Chad Lindly states: “Owning a plumbing service company here in Seattle, I can confirm that rats live in many sewers, both public and private. We have often seen them when someone has forgotten to re-install a cleanout plug or airtight fitting after cleaning a drain line. Unfortunately, these small spaces give easy access to rats.”
Dave also mentions that older sewer systems with deteriorated pipes and fittings often have small holes in pipes that offer an easy entrance for vermin.
When you are doing home repairs on your plumbing, Dave recommends extra vigilance in reassembling the system, making sure every hole is plugged. If possible, he suggests using a professional plumbing service since they will have the expertise, equipment, and supplies to effectively finish the job.