In 1987, the residents of Wilkinsburg, Pa., prepared to dig up a time capsule that had been buried in the last century. But no one could remember where it was.

In 1976, during the American bicentennial, a cross-country wagon train collected the signatures of a reported 22 million Americans to be buried at Valley Forge, Pa. But when President Ford arrived for the sealing ceremony, someone had stolen the capsule from an unattended van. It remains missing.

In 1939, MIT engineers deposited a brass time capsule beneath an 18-ton magnet used in a new cyclotron. No one has figured out how to retrieve it.

(Thanks, Luke.)