Oglethorpe University in Atlanta has a very interesting list from the International Time Capsule Society which describes the 10 most wanted time capsules. It seems that since the list's creation in 1991, two have been found.
1. Bicentennial Wagon Train Time Capsule
This capsule was supposed to hold the signatures of 22 million Americans. But on July 4, 1976, when President Gerald Ford arrived for the sealing ceremony in Valley Forge, Pennsylvania, someone stole the capsule from an unattended van in the bicentennial wagon train. The capsule's maker, the Reynolds Company, had broken the mold. The thief’s identity and the whereabouts of the capsule are unsolved mysteries.
2. MIT Cyclotron Time Capsule
In 1939 a group of MIT engineers placed a brass capsule beneath an 18-ton -magnet used in a brand new, state-of-the-art cyclotron. The capsule was to be opened in 50 years but was not. No one remembered the time capsule was there (the cyclotron had long since been deactivated). But when reminded of its existence, MIT was faced with another problem: how do you get a time capsule out from under a 36,000-pound lid?
3. Corona, California, Time Capsules
The City of Corona seems to have misplaced a series of 17 time capsules dating back to the 1930s. Efforts to recover the capsules in 1986 were in vain. "We just tore up a lot of concrete around the civic center, "said the chairman of the town's centennial committee. A Los Angeles Times reporter has called Corona "the individual record holder in the fumbled time capsule category."
4. The M*A*S*H Time Capsule
Buried by cast members of the hit TV show in a secret ceremony, the capsule contained props and costumes of the show. It was buried in January 1983 -- somewhere, no one will say -- in the 20th Century Fox parking lot in Hollywood. The lot has shrunk in size, so the time capsule may be under a Marriott Hotel now. Update: According to CNN, Alan Alda recounts in his book, Never Have Your Dog Stuffed, that the M*A*S*H time capsule was found by a construction worker shortly after the show ended. When the worker tried to return the capsule to Alda and the rest of the M*A*S*H cast, Alda told him to keep it.
5. George Washington's Cornerstone
Today's custom of burying time capsules is in part an outgrowth of Masonic cornerstone-laying ceremonies. Through the centuries, Masons have officiated at rituals which often include placing memorabilia inside building cornerstones for later recovery. In 1793, George Washington, a Mason, performed the Masonic ritual upon the laying of the original cornerstone of the U.S. Capitol. Over the years, the Capitol has undergone extensive expansion, remodeling and reconstruction, but the original George Washington cornerstone has never been found. It is unknown whether there is anything inside of it.
6. The Gramophone Company Time Capsule
In 1907, Hayes, Middlesex, England, sound recordings on disc were deposited behind the foundation stone of the new Gramophone Company factory (later HMV, later EMI) by the opera singer (later Dame) Nellie Melba. During reconstruction work in the 1960s, the container was officially removed, but before it could be reburied, someone ran off with it. The whereabouts of these priceless master-pressings of Melba and other stars remains a mystery.
7. Washington Territorial Centennial Time Capsule
In 1953 Washington state celebrated its territorial centennial by burying a two-ton time capsule on the state capitol campus in Olympia. The legislature failed to approve funds to mark the site, and the capsule was lost until 1959. However, records indicate that a supplementary time capsule was prepared in 1953 for burial alongside the main capsule. The location and contents of the second capsule are unknown. The capsule may have been interred as planned; its reported location was a closet at the capitol. Update: it appears that this capsule was found in 2002.
8. Blackpool Tower
In Blackpool, Lancashire, England, a foundation deposit was interred in the late 19th century with the customary ceremony. When a search was organized recently in preparation for new building work, not even remote sensing equipment or a clairvoyant could locate the time capsule.
9. The Lyndon, Vermont, Time Capsule
First mentioned in an 1891 Vermont newspaper, the capsule is an iron box containing proceedings of the town's centennial celebration. It was scheduled to be opened on July 4, 1991. Citizens have looked in the town vault, the bank and the library but have not found the box. The time capsule may not have been buried at all, since some ceremonies were canceled due to rain. Lyndon residents have vowed not to lose their new time capsule which is set to be sealed July 4.
10. The Kingsley Dam Time Capsule
This time capsule is only listed as "found" on the Oglethorpe site.