Mercury!

Back when I was a little kid, I read in a book of some kind about a higher-efficiency power plant that was to use a mercury-vapor topping cycle combined with a regular steam bottoming cycle. Mercury would be boiled at high temperature but low-ish pressure, used to run a turbine (that's got to be a weird turbine design) and then condensed back to liquid at still quite high temperature, boiling water to steam in the mercury condenser. That steam would then run a normal steam turbine and be condensed at ambient temperature in the usual way. Now, even as a small child, this didn't seem like a great idea to me. By this time, back in the early eighties, safety had been invented. Giant mercury boiler? You'd be better off living downwind of an RBMK.

It was with delight that just yesterday I encountered the weird and wonderful website The Self Site with its Museum of RetroTech. It turns out that several such power plants were actually built, back in the days before safety had been invented. There are even photos of what's left of one in the Mercury page.