One of the oddest electrical technologies of the past is the sodium cable. As the name indicates, it's an insulated power cable with metallic sodium as a conductor, in place of the usual copper or aluminum. Apparently it was "the next big thing" for a while in the 1960s, but as far as I know, it never caught on - though it probably "caught fire" a few times!
On pages 24-26 of this presentation, there's information about sodium cables and a cutaway photo of
The entire presentation is quite interesting; check out the "Blues Brothers" on page 19.