Making a Mammoth splash: The longest water-coaster in the world. These are the first images of the world's longest water-coaster - a ride that is guaranteed to make a splash when its (flood) gates open next year. The Mammoth, which cost $9 million (£5.5 million) and features a five-storey drop, will open at Holiday World & Splashin' Safari, in Santa Claus, Indiana, in May 2012.