A few layers down on the Blitzortung site is the following page that gives more information on how they derive their data. It's evidently a cooperative network of people who link VLF sensors via the internet. Kinda cool.
I have used the network when overseas, especially in equatorial areas to see what sort of day I can expect in the antenna fields. And it's also fun to use when DXing on the low bands to see what sort of noise wall we are listening through.
http://en.blitzortung.org/cover_your_area.php