Time capsules can be classified into two types: intentional and unintentional . Intentional time capsules are placed on purpose and are usually intended to be opened at a particular future date. Unintentional time capsules are usually archaeological in nature. Discoveries of cultural significance are often found in standard archaeological digs as well as those from volcanic eruptions such as Pompeii and Vesuvius.
The concept of time capsules is not recent. The Epic of Gilgamesh, among humanity's earliest literary works, begins with instructions on how to find a box of copper inside a foundation stone in the great walls of Uruk - in the box is Gilgamesh's tale, written on a lapis tablet. There were other time capsules 5,000 years ago as vaults of artifacts hidden inside the walls of Mesopotamian cities.
What is now thought of as a "time capsule" has more recent origins. In 1937, during preparations for the 1939 New York World's Fair, it was suggested to bury a "time bomb" for 5,000 years (until 6939)—the less inflammatory name of "time capsule" was suggested, and the name has stuck since. The 1939 New York World's Fair time capsule was created by Westinghouse as part of their exhibit, It was 90" long, with an interior diameter of 6.5 inches, and weighed 800 pounds,. Westinghouse named the copper, chromium and silver alloy "Cupaloy", claiming it had the same strength as mild steel. It contained everyday items such as a spool of thread and doll, a Book of Record (description of the capsule and its creators), a vial of staple food crop seeds, a microscope and a 15-minute RKO Pathe Pictures newsreel. Microfilm spools condensed the contents of a Sears Roebuck catalog, dictionary, almanac, and other texts. This first modern time capsule was followed in 1965 by a second capsule at the same site, but 10 feet to the north of the original. Both capsules are buried 50 feet below Flushing Meadows Park, site of the Fair. Both the 1939 and 1965 Westinghouse Time Capsules are meant to be opened in 6939. More recently, in 1985, Westinghouse created a smaller, Plexiglas shell to be buried beneath the New York Marriott Marquis hotel, in the heart of New York's theater district. However, this time capsule was never put in place.
The Crypt of Civilization (1936) at Oglethorpe University, scheduled to be opened in 8113, is generally regarded to be the first successful implementation of a modern time capsule, although it was not called a time capsule at the time. George Edward Pendray is responsible for coining the term "time capsule."
During the socialist period in the USSR, many time capsules were buried with messages to the people who would live in the future communist society.
Currently, four time capsules are "buried" in space. The two Pioneer Plaques and the two Voyager Golden Records have been attached to spacecraft for the possible benefit of spacefarers in the distant future. A fifth time capsule, the KEO satellite, will be launched in 2009 or 2010, carrying individual messages from Earth's inhabitants addressed to earthlings around the year 52,000, when KEO will return to Earth.
The International Time Capsule Society was created to maintain a global database of all existing time capsules.
According to time capsule historian William Jarvis, most intentional time capsules usually do not provide much useful historical information: they are typically filled with "useless junk", new and pristine in condition, that tells little about the people of the time. By comparison, Pompeii contains a wealth of material about daily life, such as graffiti on walls, food in hearths and the remains of people trapped under volcanic ash. Many time capsules today contain only artifacts of limited value to future historians. Historians suggest that items which describe the daily lives of the people who created them, such as personal notes, pictures, and documents, would greatly increase the value of the time capsule to future historians.
If time capsules have a museum-like goal of preserving the culture of a particular time and place for study, they fulfill this goal very poorly in that they, by definition, are kept sealed for a particular length of time. Subsequent generations between the launch date and the target date will have no direct access to the artifacts and therefore these generations are prevented from learning from the contents directly. Therefore, time capsules can be seen, in respect to their usefulness to historians, as poorly implemented museums.
Historians also concede that there are many preservation issues surrounding the selection of the media to transmit this information to the future. Some of these issues include the obsolescence of technology and the deterioration of electronic and magnetic storage media, and possibly language problems if the capsule is dug up in the distant future. Many buried time capsules are lost, as interest in them fades and the exact location is forgotten, or are destroyed within a few years by groundwater. A proposed deep time capsule The Ozymandias Project addresses many of these issues.