A conceptual problem related to a design I'm working on: If I use a pump to suck air out of the top of a vertical pipe, with the bottom of the pipe in water, I can only make the water rise around 10 metres inside the pipe, if I produce a perfect vacuum. Now let's say the pipe was 100m long and around 90m was under water, such that my pump was just able to suck the water up to the top of the pipe (and thus remove all air from inside the pipe). Now let's say the top of the pipe has a perfect one-way valve which prevents the water column from falling once I switch off the pump. Once the pipe is full of water, I turn off the pump and the column remains standing. Now, say the surrounding water level outside the bottom of the pipe falls by around 1m, such that the top of the pipe is now 11m above the surrounding water level. The perfect one-way valve at the top remains closed. What happens inside the pipe? Will my column of water still be standing, reaching to the top of the pipe? What governs this behavior and, if the water column can be more than 10m in this situation, what happens if the water level drops significantly further?