"Suicide By Squats", a brutal "How long can you last?" exercise.
Blog writeup: http://regex.info/blog/2017-09-23/2825
Set a timer. During the first minute, do two deep, proper-form squats. Do four during the second minute, six during the third, and so on, adding two for each minute. When you fail to be able to do the squats for that minute, you're done.
I'd just done it on Thursday and was still sore, but thought "what the heck?". I didn't think I'd make it all that far, so I was surprised to make through minute 19. I might have made the next minute as well (as I did on Thursday) but at the end I started to get a weird throbbing pain in my head, so I just stopped.
Not sure I believe the bump in the HR graph around minute 14.... I think I would have noticed that.
I measured my weight with a digital scale before and after. I lost 1.05kg (23lb) in sweat doing this exercise.