Nasa releases cool pics of earth at night from space

Check it out:

[link to www.nasa.gov]

I only have one question though...

If you go straight north from Denver toward Canada you will see a huge lit up area about the size of Chicago.

I matched it up on google maps. It appears to be the area in and around the Fort Berthold indian reservation in North Dakota. Wikipedia says that reservation has about 3,700 people only.

So why is it lit up like a christmas tree?