Flooding hit Houston Monday night, washing out roads and swamping neighborhoods. A flash flood emergency was issued. The Associated Press said emergency crews handled more than 500 water-related calls for help, most from drivers stuck in their vehicles.

Flooding is a threat people often ignore until it's too late. But after seeing these eight photos from the nightmare in Houston, we think you'll understand why you need to be so cautious. 

1. Lightning hitting a building, illuminating the floodwaters that swamped the town.

2. This before-and-after shot of I-45 Northbound.

3. Floodwaters swamping everything, as far as the eye can see.

4. The entire bottom floor of The Galleria's parking deck was submerged.

5. Houston's traffic map Tuesday morning.

6. This person who took on the widespread flooding in a little red canoe.

7. Cars piled up like toys along the side of I-45.

8. Bridges with a 14-foot clearance were still nearly flooded.

MORE ON WEATHER.COM: A Bunch More Images Of The Southern Plains Flooding