On September 15, 2002, about 1947 eastern daylight time, a Cessna 421, N23RL, was substantially damaged while landing at the Ithaca Tompkins Regional Airport (ITH), Ithaca, New York. The certificated commercial pilot, the certificated airline transport pilot, and the three passengers were not injured. Instrument meteorological conditions (IMC) prevailed for the flight that departed the Morristown Municipal Airport (MMU), Morristown, New Jersey, about 1845. An instrument flight rules flight plan was filed, and the personal flight was conducted under 14 CFR Part 91. According to the pilot he arrived at the airport about 1810, and preflighted the airplane while the second pilot obtained the ATIS and IFR clearance. The pilot occupied the left seat, started both engines, and taxied to runway 23. Once airborne, he climbed the airplane to 12,000 feet, and the flight progressed without incident. In the Binghamton, New York, area, the pilot intercepted the localizer and glide slope for runway 32, and initiated a descent. He engaged the autopilot while obtaining the ATIS and briefing the approach to the second pilot. The pilot then disengaged the autopilot, and continued tracking the localizer inbound. Before reaching the locator outer marker, the pilot lowered the landing gear, and selected approach flaps. During the approach, the airplane was in IMC until reaching about 700 feet agl. Once clear of the clouds, flight visibility was approximately 2 miles, the airplane was in light rain, there was no turbulence, and both pilots could clearly see the airport. The pilot was advised by the tower controller that it was raining heavily at the airport. The pilot then selected landing flaps, and slowed the airplane to approximately 100 knots. The intensity of the rain increased, and the airplane touched down "smoothly" near the runway aiming points and "slightly" left of centerline. The pilot lowered the nose, and "lightly" applied the brakes. The airplane then started to hydroplane, and yaw left. It exited the left side of the runway, and slid several hundred feet before coming to a stop. According to a Federal Aviation Administration inspector, the airplane came to rest about halfway down runway 32, and off the landing surface to the left. The runway was 6,601 feet long, 150 feet wide, and surfaced with asphalt. Examination of the wreckage revealed the left main landing gear had separated from the airplane, and the left horizontal stabilizer was substantially damaged. Examination of the wreckage revealed no preimpact failures or malfunctions.