it's surly not steam, the water in the pipe expands and pushes out of each end, or more likely just one, as there's no way for air to get in it just pulls more cold water through, which heats and the cycle continues

would it not possibly create shadowing from the squared edges?

i wish people didn't view myth buster's experiments as the be all and end all of science, they are often poorly thought out, and rarely cover ever anomaly that could come up in the experiment.

there was a British show for the open university presented by Adam hart davis, possibly 'what the ancients did for us' (or one of his other shows) that demonstrated that in ancient times it was possible to have done this using available resources. whether or not it was successfully achieved is another story, but they showed that it was entirely possible. they even went to Greece to prove it.