Equip the edges of the lattice ℤ2 with i.i.d. random capacities. A law of
large numbers is known for the maximal flow crossing a rectangle in ℝ2 when the
side lengths of the rectangle go to infinity. We prove that the lower large deviations are
of surface order, and we prove the corresponding large deviation principle from below.
This extends and improves previous large deviations results of Grimmett and Kesten  obtained for boxes of particular orientation.