I was just riding my normal circuit and the bike had a wobble that was unusual. I stopped a couple of times, but nothing looked out of place, no wheel or tire issues - nothing obvious. I was stopped at a light, and got up out of the saddle, did a few revs and CLUNK. Luckily I was in a bike lane with a curb/sidewalk. At first I could see it, then I saw the down tube completely broken in two. I'll upload a photo of the break in one of my other answers.