These giant amoeboid creatures look like nothing more than
puddles of slime, but they both live and move, feeding on
metal or wood as well as the occasional dungeon explorer to
supplement their diet.

But we were not on a station platform. We were on the track ahead
as the nightmare, plastic column of fetid black iridescence oozed
tightly onward through its fifteen-foot sinus, gathering unholy
speed and driving before it a spiral, re-thickening cloud of the
pallid abyss vapor. It was a terrible, indescribable thing vaster
than any subway train -- a shapeless congeries of protoplasmic
bubbles, faintly self-luminous, and with myriads of temporary eyes
forming and unforming as pustules of greenish light all over the
tunnel-filling front that bore down upon us, crushing the frantic
penguins and slithering over the glistening floor that it and its
kind had swept so evilly free of all litter.
[ At the Mountains of Madness, by H.P. Lovecraft ]