Oobleck, a non-newtonian fluid made with cornstarch and waterThe artists who created this installation are Alex, Mary, and Max.
Corn strarch and water (plus some optional green food coloring) creates a strange thick non-newtonian fluid called Oobleck. When stress is applied to the liquid it exhibits properties of a solid, i.e. its viscosity increases tremendously. A hammer will bounce on it, but you can sink your hand into it with no effort if you do it slowly. it's really fun to play with, and you can do the mix in your kitchen! (i discovered the strange properties of this mix when i was a kid, playing in the kitchen).
Check this very amusing video showing what you can do with a pool filled-up with this liquid. Just like Jesus, you can walk on the liquid!
More info on Wikipedia.
Photo taken at the Burning Man 2007 festival (Black Rock Desert, Nevada).