The river of slime- from Ghostbusters II (1989)
When the Ghostbusters examine the sewers under their beloved New York, they find an ominous river of pink slime - mood slime as it's known in the film.
The shot is comprised of three elements: The actors were shot on a limited live-action set. The river of slime is a miniature, 1 feet wide and 10 feet long. It was built with a slight tilt, to allow gravity to take care of the flow. The slime itself was Methocel, mica dust, food dye, and mineral oil. It looks fantastic, and completely out of this world, which I guess it is.
Finally the rest of the image is an Yusei Uesugi matte painting. He painted everything, including the grid floor on the right, the tunnel, and the whole left section of the wall!
There's something wonderfully rough and organic about the effects for the Ghostbusters movies, and this shot perfectly demonstrates that. Not every shot looks completely convincing, but since we're dealing with a world of goblins and spooks it's okay if things are a bit "off". They're still magical.