It's been nearly 10 years since Tropical Storm Allison devastated the Houston area with 40 inches of rain. The floodwaters shut down hospitals, destroyed thousands of homes and killed 23 people in Texas. The storm caused flooding the first time it passed over the bayou city on June 4, 2001. But the worst flooding came a few days later when it turned around and targeted Houston again. Allison caused $5.5 billion in damages and taught Houstonians that tropical storms can be even more dangerous than hurricanes. It remains the deadliest and costliest storm in U.S. history.