Gizmodo has a neat article about vertical cemeteries, some hypothetical and some existing, which are designed to solve the problem of land scarcity for cemeteries. The United States does not generally suffer from land scarcity except in very developed urban areas like New York City. We therefore have a strong social norm of perpetual dedication of a gravesite. One person, one grave, forever and ever. (Or at least until someone wants to build a Walmart on the site, or expand a road or runway.) This idea of perpetual dedication is consistent with the extreme lengths we go to in order to "protect" human remains from decomposition: embalming, burial in a metal "leak proof" coffin, and encasement in a vault. (Of course, none of these measures actually prevents decomposition.)
But in more densely populated countries which still practice widespread ground burial, finding land for the dead is a problem. The solution practiced in a number of European countries is "grave recycling." A corpse is buried in a manner that encourages decomposition (no metal coffins and vaults), and after a set period of time (usually 20-50 years), whatever is left is disinterred and deposited in a communal ossuary at the cemetery. The grave is then turned over to the next temporary inhabitant.
Although this idea is foreign to most modern Americans, we have a limited historical tradition of grave recycling. In 1823, there were more than 600 vaults in lower Manhattan that each held the remains of 10-20 people. Some still exist. The best example of dealing with land scarcity in America has been New Orleans. The photograph above is of a wall vault at St. Louis Cemetery #1 in New Orleans These wall vaults, which line the perimeters of the older New Orleans cemeteries, were generally rented by the Church for a year and a day. After that period, the remains were removed, bagged, and placed in a commual ossuary. Even the family vaults and in-ground graves in New Orleans cemeteries feature grave recycling. The photograph below is of family in-ground graves in Metairie Cemetery in New Orleans, each of which hold multiple remains.
h/t Ryan Radford