American Civil War dead are threatening the living, according to Smithsonian. It's not a zombie thing, but a problem of arsenic used in some corpse embalming procedures.
“A Civil War-era cemetery filled with plenty of graves—things seldom stay where you want them to,” says Benjamin Bostick, a geochemist at Columbia University. "As the body is becoming soil, the arsenic is being added to the soil.” From there, rainwater and flooding can wash arsenic into the water table.
That means old cemeteries full of deceased soldiers and civilians present a real problem for today’s homeowners. The federal government says it's only safe for us to drink water with 10 parts per billion of arsenic or less. But in 2002, a USGS-sponsored survey in Iowa City found arsenic levels at three times the federal limit near an old cemetery.
“When you have this big mass of arsenic, there’s enough to affect literally millions of liters of water at least a little bit,” Bostick says.
(many thanks to Amanda Lee)