What is the biggest fire to burn in the US? The answer requires a journey through history.

Evidence found at the heart of centuries-old trees, in the stories of Indigenous peoples and yellowed journals and newspaper clippings bear record of the massive fires that ravaged landscapes across North America in the past.

But comparing those fires to modern day blazes such as the Smokehouse Creek fire in Texas in February isn't as simple as it sounds. Past records, when they're available, can be conflicting, haphazard and they rely on anecdotal observations or measurement techniques that may not be as precise as today.

Cowboys riding the range on horseback in the 1800s to measure a blaze isn’t quite the same as measuring by satellite the acres burned.

To develop a comprehensive list of the nation's biggest fires, USA TODAY combed through historical records from state and federal agencies and the Western Fire Chiefs Association and talked with several fire historians and experts. We also compared lists and documentation with Birgitte Messerschmidt, director of research at the National Fire Protection Association. 

The documents and conversations produced a list of 35 wildfires larger than 500,000 acres in 19 states, dating back to 1825. Nearly half the fires burned in Alaska, including seven before it became a state. These are the fires that covered the most ground, not the most deadly or the most devastating in terms of structural loss and damage. 

What are the biggest wildfires in US history? 

The full list is online here. The following are the 20 largest fires in the United States, although several of the fires in Alaska, and one in Oregon, were before statehood. The fires are ranked by estimated acres burned:

1. Great fire of 1910, Idaho and Montana – 3,000,000 
2. North and South Carolina wildfire complex, 1898 – 3,000,000
3. Great Michigan Fire, 1871 – 2,500,000
4. Great Fire, Oregon, 1845 – 1,500,000
5. Taylor Complex Fire, 2004. Alaska – 1,303,358
6. Peshtigo Fire, 1871, Wisconsin – 1,250,000
7. Ruby Fire, 1940, Alaska – 1,250,000
8. Kateel River, No. 5, 1957, Alaska – 1,161,200
9. Smokehouse Creek Fire, 2024, Texas – 1,058,482
10. August Complex, 2020, California – 1,032,648
11. Thumb Fire, 1881, Michigan – 1,000,000
12. Unalakleet, 1941, Alaska – 1,000,000
13. Dixie Fire, 2021, California – 963,309
14. East Amarillo Complex, 2006, Texas – 907,245
15. Little Black River Fire, 1950, Alaska – 892,900
16. Lime Complex, 2022, Alaska – 892,900
17. Miramichi Fire, 1825, Maine – 832,000. (Another 3 million acres burned in New Brunswick, Canada.)
18. Solstice Complex, 2004, Alaska – 812,771
19. Holanada Creek Fire, 1969, Alaska – 812,771
20. Northwest Oklahoma Complex, 2017 – 779,292

What to know about the largest wildfires

Any list of historic fires comes with a few footnotes. Some of the fires on the list were multiple fires that burned together, others were single fires.

The Smokehouse Creek Fire was considered a single fire, until the Reamer fire – roughly 2,000 acres in size – was added to it, according to Texas A&M officials. When only fires in the lower 48 states are considered, it was the biggest single fire in more than a century.

It's certain that fires are missing from the list because they weren't reported at the time, possibly because they didn't really affect towns or many people, said Jennifer Marlon, a senior research scientistat Yale School of the Environment.

Research shows the 1889 fire season in the Rockies and the Northwest exceeded the area of the 1910 fire season, for example, but no agencies reported numbers, said Jed Meunier, a fire historian and research scientist with the Wisconsin Department of Natural Resources.

In 1891, a large fire burned again in the region where the Thumb fire in Michigan burned ten years earlier. Based on tree ring evidence it was one of the biggest fires in the region, Meunier said. But only merited one line in a historic timeline and no other records about the fire are readily available.

Though they don't appear on the Texas state list of large fires in history, historical accounts show grass fires in Texas and New Mexico in 1906 may have exceeded 6 million acres, said Stephen Pyne, an emeritus professor at Arizona State University and author of several books on fire history. A 1967 paper by a Texas Tech professor reported a fire in eastern New Mexico burned a strip 150 miles long and 60 miles wide, and a fire on the Texas high plains later that year that burned a million acres.

Measuring the biggest wildfires

Each era has had its own method of recording and measuring fires, Pyne said. Fire mapping methods changed over time.

In the Adirondacks in 1903, a map was drawn for an official report and in 1910, a Forest Service cartographer prepared an official map of the fire and smoke.

The first big difference in fire mapping happened "when we started being able to map with aircraft around the late 1950’s/early 1960’s," said Jim Karels, president of Karels Forestry and Fire LLC, who previously served as the national fire director for the National Association of State Foresters and as director of the Florida Forest Service. "Then came GPS in the 2000’s and finally now with satellite imagery, including infrared for seeing through clouds or mapping at night, for large fires."

Today's mapping is much more accurate and can be updated daily or even hourly, Karels said. "We are even able to map burn severity within the fire itself."   

Better fire mapping also tends to make the total size of the fires smaller than some of the big fires of the past, he said, because the calculations take out the unburned pockets.

US fires and smokeSee the latest fires in this interactive map

The value in understanding fire history

One thing is certain, large wildfires definitely aren't new, the fire historians said. Many landscapes across the continent evolved to depend on fire for forest health.

Incredibly massive wildfires burned across the North American landscape long before European settlers arrived, said Marlon and Meunier. They know that millions of acres burned by studying tree rings and paleo records, then compiling estimates of fire seasons or the approximate size of acreage burned in a single season. The U.S. Forest Service reported as early as 1912 that tree rings showed fires in California more than 1,600 years earlier.

Though there were large fires during the settlement era, they were not as frequent.

In the 20th century, modern firefighting efforts kept the larger conflagrations at bay for nearly a century. Today, large wildfires are back, Pyne said. “Megafires have returned in a big way."

The chances of large wildfires are increasing with climate change, and conditions are becoming more unpredictable, Marlon said. For example, temperatures are rising, flash droughts occur more often and hurricanes occur more often farther north, leaving downed trees in their wake to provide fuel for fires.

Studying the wildfires of the past has value because it shows any region of the country could experience a massive wildfire, she said. "People think it's just something that happens in the West or the South," she said. "We've had bad fires (in the Northeast). It could happen again."

Dinah Voyles Pulver covers climate and the environment for USA TODAY. Reach her at dpulver@gannett.com or @dinahvp.