Most cemeteries insist on an outer container, which can be unlined concrete, or a sealed vault, either metal or concrete. The unlined concrete will let water and "cemetery elements" in, like bugs and the like, so the casket will still rot quickly, as will the contents.

In STL, Catholic cemeteries don't insist on an outer container, and will even reuse a grave after 40 years, if a wood casket was used with no container.

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2002 Ford ZX2
2 x 2013 Honda Civics