The palaces and even the fountains themselves aside, the most impressive engineering in Peterhof is the hydraulics. In just eight months, Peter I had a 20 kilometre waterway with distribution sluices dug, including the canal that runs from the base of the Great Palace's terrace to the sea. Of the eventual 173 fountains, not a single one operates on a pump system. All of the jets, water arches, sprays and tranquil pools are courtesy of the pressure created by natural elevation and all of the water is from the natural springs lying just below the upper gardens. The tour of the grotto, located just behind the grand cascade includes an explanation of the remarkable technology and also gives a bit of insight as to Peter's sense of humor – reaching for the bowl of fruit at the central table will result in a surprise soaking. Oh, that Peter.