The big stain that's been forming lately - partly on the ceiling of our bathroom, and partly on that of our study - began to worry me. This morning I called a plumber, who climbed up to the attic to search for a probable cause. I thought it was either a dislocated roof tile, or a busted water pipe. The answer was much simpler, and much cheaper to fix: a clogged a/c drain (the thin white tube on picture #3). He told me to try and lift it, and it fealt heavy like lead. "Get a new drain and you're good", he said. The stuff you go through when you own a big concrete box...