Firefighters from around the country began arriving in California to help battle some of the largest wildfires in the state's history.There are more than 550 fires burning throughout the state, but three clusters of fires in Northern California are causing the most damage.Together, these three blazes, which consist of dozens of fires each, have scorched more than 1,045 square miles as of this morning.There are currently more than 12,000 people helping fight the fires. Each of the three big blazes is 15 percent contained or less.So far, more than 500 structures have been destroyed, more than 100,000 people are under evacuation orders, and at least six people have been killed.The fires were sparked by lightning earlier this week. Forecasters said there was a chance of more thunderstorms this weekend, which could bring wind and more lightning.The massive amounts of smoke and and ash from the blazes were visible on satellite images, and is making the air unhealthy to breathe around the San Francisco Bay Area.For more, we caught up with Bay Area News Group reporter Julia Sulek yesterday.She was in the town of Bonny Doon, northwest of Santa Cruz,where the CZU lightning complex fire had burned 57,000 acres, and was only 2 percent contained at the time we spoke.You've gone from fire to fire in this region. Why is this happening so frequently?Well, you know, here in California, everything is just so dry. And this has been a very unusual fire.There are so many that were sparked by lightning and lightning in California is such a thrill for northern Californians is because we never see it.