There was a great article on nrk.no about how people are still not decomposed in their graves after 70 years: https://www.nrk.no/viten/xl/plassmangel-og-blaleire-gir-stor...

One example is clay. Burying in clay rich ground will slow decay because it limits the flow of air.

Another example is from the late 1950s where people would be buried in plastic wrap, to make the burial process more sanitary. They did this for 15-20 years. None of those bodies have decomposed much, but worse is that all the liquids are trapped inside the plastic :|