Next week is the anniversary of the start of the the Tubbs Fire, which killed 22 people and burned more than 5,000 homes and other structures in Napa, Sonoma and Lake counties, including 2,800 homes in Santa Rosa.

Wildfires have been part of California’s natural landscape for millions of years. They clear out dead brush and trees, release nutrients into the soil and help some plants grow, by releasing seeds.

But with California’s population expanding into fire-prone areas, the climate getting warmer and fire fighters having spent a century putting out fires, the risk to human lives and structures continues to increase.

Cal Fire, the state’s main firefighting agency, has built a database over the years showing the perimeters of major fires in California. Although the list is not complete, because early records are imperfect, the state and U.S. Forest Service have documented more than 25,000 large fires since 1900, which have burned 35 million acres — an area equal to about one-third of the land in the state.

Many of the places that burned have burned multiple times over the past century. Combining all the fire perimeters onto one map shows that certain areas, including the hills around San Diego, Los Angeles and Santa Barbara, along with the Central Sierra Nevada, Big Sur, Northern San Francisco Bay and the Klamath Range, have experienced repeated and regular wildfires.