As many as 51,000 soldiers died in the pivotal Civil War battle commemorated here, and even though the battle ended 150 years ago, some continue to wage war beyond death. In addition to hearing phantom gunshots and drum rolls throughout the park, visitors have spotted a headless horseman at Little Round Top, sharpshooters at Devil’s Den, and various soldiers throughout the park. Vicksburg and Fredericksburg national military parks report similar activity.

The Grand Canyon is reportedly home to a whole host of apparitions. Boys run through Hopi House; Fred Harvey, whose company managed hotels and restaurants along the railroad, occasionally appears at El Tovar in a long coat and black hat; and the ghost known as Wandering Woman searches for her husband and son who died in a hiking accident. In the canyon, a worker crushed by a boulder also haunts the site near Phantom Ranch where he was buried, and eerie lights can be seen in Crash Canyon, where two passenger jets collided in 1956.

This Kentucky cave also has its fair share of ghosts. The Native Americans buried here roam the caverns – as do cave explorers, including Floyd Collins, who died after being trapped. Listen carefully, and you might hear his cries of “Help me! Help me! I’m trapped!” Or, there's Mammoth Cave’s most famous ghost, a girl who abandoned her tutor – and unrequited love – in the darkness out of spite. Feeling guilty, she went back but never found him. Her ghost allegedly continues the search.

Technically, Stanley Hotel isn’t part of Rocky Mountain National Park, but it sits on the park's edge and is arguably one of the most haunted hotels in America. Some guests, including author Stephen King, have reported a boy calling for his nanny on the second floor; others have heard children playing on the fourth floor. But the hauntings aren’t limited to guest rooms; original owner F.O. Stanley supposedly has been seen walking the lobby, while his wife can be heard playing the piano in the music room.

This park, immortalized by the photographs of Ansel Adams, has a few immortals of its own. A man hung himself in what once was Camp 6, young couple drowned at Stoneman Bridge, and ghostly Native Americans are said to linger among the pines. But for the densest area of paranormal activity, step out to mezzanine or venture up the third floor of Ahwahnee Hotel – the property served as a naval convalescence hospital during World War II.

On Sept. 11, 2001, Flight 93 crashed into a Pennsylvania field here as its passengers fought for control of their hijacked plane. In the months after the crash, security guards reported hearing footsteps in and around the temporary trailer set up on the site; one even claimed to have seen a ghostly woman in a blue baseball jersey near the front gate. He later identified her as one of the passengers on the flight.

When the USS Arizona sunk during the Japanese attack on Pearl Harbor on Dec. 7, 1941, it became the watery grave for 1,177 people. Some visitors claim to hear screams, strange noises, and alarm bells coming from under their feet at the memorial, while ghostly young men have also said to been glimpsed running on the submerged deck.