In the past 15 years, the Davenport, Wash.-based National UFO Reporting Center has collected more than 30,000 reports of UFO sightings through its hotline and Web site. The following three maps visualize that rich history.

The first plots every sighting as a small yellow dot, with lighter colors representing a higher density of reports.

The second map plots the reports week by week—just click on an icon to read a description of the sighting. In many cases, several people report what appears to be the same incident. For example, there was a huge cluster of UFO spottings in the Northeast on Sept. 19, 2009. (NASA launched a rocket experiment in Virginia that night.) Astronomical events are often misidentified as well, such as in the third week of November 1999, when several people reported seeing a UFO in the Midwest. It was the same night as the Leonid meteor shower. Reports also spike in the first week of July.

Because the first map is heavily dependent on population density, the third map shows sightings per capita in every county where data were available. (View a larger version on Slate Labs.)