Go out on the battlefield at Gettysburg in the evening, in a part where there aren't a whole lot of tourists.
It's the evening of July 3, 1863, with 50,000 men dead or wounded after grappling at close quarters.
Imagine the despondency and frustration of the Confederates, who had seen their last hope for victory in the battle and probably the war evaporate with Pickett's men under Union artillery fire at Cemetery Ridge, who had watched those comrades march open-eyed into the teeth of a storm one out of every twelve of them would die in.
Imagine the Union troops, who after having triumphed were nevertheless probably too exhausted and shell-shocked to feel much but blank apathy. They had suffered thousands of casualties of their own and had seen their fair share of horror.
And imagine the civilians, some of whose houses were makeshift hospitals ankle-deep in blood, resounding with the moans and screams of wounded men (but none of the screamers probably screamed very long... one way or the other).
If any place on earth is haunted, Gettysburg surely is -- even if the ghosts there walk only in our own minds.