You may know that a pump can’t lift water more than around 10 metres. This is because it is atmospheric pressure that pushes on the water to force it up the pipe as the pump removes water from the top. But the pump can’t create pressure lower than zero, so there’s a limit set by ambient atmospheric pressure (lower at high altitudes). But what happens if you try? The water boils. At some point, the pressure in the pipe falls sufficiently low that the boiling point falls below the temperature of the water and it begins to boil, filling the gap above the column of water with water vapour. At a pressure of 6.1 millibars, the boiling point of water falls all the way to 0° C.