water stain on ceiling 4

https://brightquillio.com/s/WITUIB2HHvy9XsxdEHiwp

There's this pale brown blotch spreading across the ceiling above the bathtub, shaped vaguely like a continent on an old map. It started as a tiny spot, but every time it rains, it grows a little bigger. Now it's about the size of a dinner plate, with wavy edges and a darker center, almost like it's radiating from the pipe joint. I keep meaning to scrub it off, but it's actually kind of fascinating to watch, and anyway, I'm pretty sure painting over it won't solve the leak.