Throughout history, many soldiers have died in wars without their remains being identified. In modern times, nations have developed the practice of having a symbolic Tomb of the Unknown Soldier that represents the war grave of those unidentified soldiers. They usually contain the remains of a dead soldier who is unidentified (or "known but to God" as the stone is sometimes inscribed) and thought to be impossible ever to identify, so that he might serve as a symbol for all of the unknown dead wherever they fell. The anonymity of the entombed soldier is key to the symbolism of the monument: since his or her identity is unknown, it could theoretically be the tomb of anyone who fell in service of the nation in question, and therefore serves as a monument to all of their sacrifices. Much work goes into trying to find a certain soldier, and to verify that it is indeed one of the relevant nation's soldiers. See Wikipedia for national examples.