A modern-day record number of visitors went underground at Wind Cave National Park in 2015/NPS

A number of National Park System units saw record visitation last year, and among them was Wind Cave National Park in South Dakota, where more than 109,000 visitors went underground.

That number, 109,335, reflects a nearly 10 percent increase in cave tours over 2014, according to park staff. It also marks the most people through the cave since 1971, before tours were limited to 40 people.

“This year the National Park Service is celebrating its centennial, and we’re encouraging everyone to Find Your Park. Many people found their park here last year, and we’re looking forward to the possibility of breaking this record again this year,” said Wind Cave Superintendent Vidal Dávila.

Rangers, park guides, and interns led 3,769 tours through Wind Cave last year for visitors from throughout the world. Just more than 70 percent of the visitation occurred between June 1 and September 1. The busiest days were Tuesdays and Wednesdays and any day that it rained.

In an era when tour sizes were not limited, 1968 saw the highest annual cave tour visitation with 138,348 people through the cave, according to park records. The park’s largest tour on record, 210 people, occurred in 1966 on a tour that nowadays would have been limited to 40 people. Because of limiting tour sizes and rerouting Highway 385 to by-pass the park visitor center, park staff believe the historic 1968 figure will never be exceeded.