South Dakota officials are inspecting seven scenic tunnels in the Black Hills this week for structural integrity.
The Rapid City Journal reports that engineers and a geologist are monitoring the tunnels on Needles Highway and Iron Mountain Road for weathering and erosion. The tunnels were blown out of granite with dynamite in the 1920s.
The inspection comes as the Federal Highway Administration requires more routine checkups every two years.
South Dakota officials aren't concerned the tunnels will collapse, but they will gradually erode, just like any rock shelf.
Bridge maintenance engineer Todd Thompson says inspection crews try to monitor the tunnels in the fall to minimize road closures' impact on Custer State Park. Millions of travelers take road trips to the Black Hills in the summer.