These dudes from the Spanish TV show El-Hormiguero mixed cornstarch and water made on a concrete mixer truck, effectively creating a pool of non-newtonian fluid – which is almost like normal water, but when stress is applied to the liquid it exhibits properties of a solid.
I don't think you would have bumped – quite literally in this case – into a such a material in old nature. Well, maybe in a swamp. But that is less fun. Just imagine all the next Olympic sports we could build on this! Hypermaterial of the moment.