The outside drain has been blocked recently, leading to the backyard getting half-flooded any time someone has a bath. For some reason, anything to do with plumbing and DIY round here is my job (David: “I’m cute! I’m pretty! I put out!”), so armed with caustic soda, a bundle of drain rods, extra-thick rubber gloves and grim determination, I tackled it.
And found a massive wodge of matted, soggy, incredibly smelly… HAIR. About half human, half husky. It had formed into a fairly compacted ball in the bottom of the drain, where a whole load of decomposing… STUFF got worked into it.
tl;dr: The drain had a bezoar.