For anyone interested in a demonstration they did this on the Mythbusters Breaking Bad special. They had a lot of difficulty getting it to work properly (it’s certainly not as simple as depicted in Breaking Bad, where the simplicity of body disposal in those barrels intentionally became a joke), and my feeling is that most murderers would not be able to pull it off. I only read the Wikipedia article on Haigh but it seems to me that his victims would not have had to be completely dissolved. He dumped the sludge after a couple of days into an open sewer - even if there was a lot of flesh and bone intact, it would have broken apart and gotten lost down there.
Anyway, @Donald_Petersen et al. - in regards to lava I do certainly think you could hide a body. However, with a small flow like this you probably wouldn’t actually disintegrate the body - the lava would cool and form a shell around at least part of the body, which you could then break open… TV/movie writers, feel free to send me a check. You’d want to get to one of those big flows that end up in the ocean, which would either burn to a crisp or scatter irretrievably every part of the body, and then bury it under a big pile of fresh basalt.
The problem really is getting the body there. Easily solved if you plan to commit your murder on-site, but then the problem is that there are almost always constantly people observing active lava flows, no matter how small. You might have to take the approach of eliminating all the witnesses, which you could pass off as being a tragic accident.
