Continental Air Lines Flight 12 departed Los Angeles on a scheduled flight to Chicago-O’Hare with an intermediate stop at Kansas City. Following an en route descent, the flight began an ILS approach to runway 18 at Kansas City After passing the outer marker the flight was cleared to land straight in on runway 18. The wind was reported to be from 070 degrees at 7 knots. During the approach the flight crew encountered light rain and light to moderate turbulence. The windshield wipers were turned on, full flaps were lowered, and the approach continued. Touchdown was within 5 knots of the airspeed specified for the gross weight, and between 1,000 and 1,200 feet past the approach end of the runway. Heavy rain was noted at that point. Immediately after a “firm” touchdown, the nose gear touched down. Brakes were then applied, the spoilers were raised, and reverse thrust was applied to 80 percent. After the aircraft rolled approximately 4,000 feet, reverse thrust was increased to 100 percent. The aircraft did not slow down as expected and went off the end of the runway cocked about 30 degrees to the left, at an estimated 40 knots. The right wing struck the blast mound and the aircraft slid up over the mound and came to rest with the nose section in the perimeter road between the blast mound and a river levee. PROBABLE CAUSE: “Hydroplaning of the landing gear wheels, which precluded braking effectiveness.”