More than 4 million visitors flocked to Yellowstone National Park last year, and most of them probably never ventured far enough off the well-trodden path to see one of the park's true wonders: the night sky.

Here & Now's Peter O'Dowd got a late-night tour of the heavens with Park Ranger Tammi Corchero, who explained why the night sky is endangered.

The Milky Way over Mammoth Hot Springs in Yellowstone National Park. (Neal Herbert/National Park Service via Flickr)

The Milky Way near Mammoth Hot Springs in Yellowstone National Park in October 2013. (Neal Herbert/National Park Service via Flickr)

The Milky Way over the Hoodoos area in Yellowstone National Park. (Neal Herbert/National Park Service via Flickr)

The Milky Way over Firehole Lake Drive in Yellowstone National Park. (Neal Herbert/National Park Service via Flickr)

The Milky Way over Castle Geyser in Yellowstone National Park in July 2016. (Neal Herbert/National Park Service via Flickr)

Guest

Tammi Corchero, Yellowstone National Park ranger. The park tweets @YellowstoneNPS.