-- What's the volume of a cylinder with radius=1m and height=55m ?
( Volume of a cylinder = π R² h )
-- How much does that volume of water weigh ?
1 liter of water = 1 kilogram of mass
Weight = (mass) x (acceleration of gravity)
-- What's the area of the bottom of that 1m-radius cylinder ?
Pressure = (force) / (area)