Go to Beadeaux. Proceed straight in, then turn left (go north) when you reach the wall at F-7. Go north to the wall in F-6, then due east to H-6. From here head south and you'll find yourself going into a cave that leads down. Take that down until it exits at H-7 on the lower map.
From here, go west and south as you can until you come to the tunnel at F-8 that leads back to the surface; you will pass one Afflictor that will Curse you as you proceed. Follow the surface path along, south past the Mute at F-9. Continue south, under the bridge and past the second Afflictor, then head east around the giant smokestacks at G-9/H-9.
When the weather is raining, a ??? will appear near the base of the smokestacks at G-10. Select that to receive the Corrupted Dirt (key item).