What Is This?

Clearly, this is a map of the Midwest. But what exactly is it showing us? Localized population densities? A hellmouth opening just south of La Crosse? Warning: The real answer might bug you out.

It's a radar image, sure. A radar image of the millions of swarming mayflies taking over Wisconsin:

A mayfly hatch along the Mississippi River was caught on Doppler radar out of NWS La Crosse this evening. The radar view below shows an image at 9:13 p.m. CDT on Saturday, May 29, 2010. The bugs are showing up as bright pink, purple, and white colors along the Mississippi River mainly south of La Crosse, WI. After the bugs hatch off the water and river areas, they are caught in the south-southeast winds while airborne for about 10-20 minutes.

That. Is. Gross. [NOAA via Neatorama]