As the National Park Service turns 100 years old in August, people are flocking to the parks, monuments, historical sites and other national treasures that the agency oversees.

Total visitors in 2014 and 2015 were higher than in previous years, according to National Park Service visitor data on about 375 sites where they track attendance or generate estimates.

An estimated 307 million people visited Park Service units last year, up from about 273 million two years earlier.

This spike is particularly apparent among national parks, such as Yellowstone, Glacier, and Minnesota's own Voyageurs National Park.

In addition to Voyageurs, the Park Service's Minnesota units include the Mississippi National River and Recreation Area and the Pipestone National Monument.

In Wisconsin, the Park Service oversees the St. Croix National Scenic Riverway, the Apostle Islands National Lakeshore, and the Ice Age National Scenic Trail.

The National Park Service doesn't quite know what's driving the new popularity, but great weather in the interior West may have played a role. It's also possible that the lower numbers in previous years might have been due to renovation closures at various sites, plus the government shutdown in 2013 that also resulted in closures.

Visits to Voyageurs National Park in northern Minnesota are similar to the national trend, except there's a big dip in 2011. A spokesperson for the park said they don't know why visitor totals are lower in some years than others, but that sometimes weather or the economy play a role.

Locally, we know that one reason for a spike in attendance in 2014 were the ice caves that winter in the Apostle Islands, near Bayfield, Wis. The cold weather that year made for optimal ice coverage on the lake so hikers could make the two-mile round trip to the caves, where visitors were dazzled by impressive ice formations.

The National Park Service provided us annual visitor data for 375 sites in the United States each year that visitors have been tracked.

The data starts in 1904 for six national parks: Yellowstone, Crater Lake, Hot Springs, Mount Rainier, Wind Cave and Kings Canyon.

In 1904, the park service estimates just over 13,700 people visited Yellowstone. Last year, there were more than 4 million.

What are the most popular sites? Last year the top three were the Blue Ridge Parkway in North Carolina (15 million), the Golden Gate National Recreation Area in California (14.8 million), and the Great Smoky Mountains National Park (10 million).

Check out your favorite sites in this searchable database and map. You can scroll through the list of sites or type in a name at the top.

Note: Sometimes you'll see the attendance drop to zero. There are a variety of possible reasons. One is that the site didn't report the number of visitors that year; another is that the site was closed for renovations; or it's possible that the site quit tracking attendance. If you're really curious, you can look at the statistical abstracts that are part of the park service's annual reports.