Imagine if the ancient Egyptians still ruled Egypt, more than half of Egypt would have been filled by pyramids
For environmental reasons, cremation (does release pollutants such as mercury and carbon dioxide into the atmosphere, but could be filtered out) is better than traditional burial.
Cremation might be preferable for environmental reasons. Burial is a known source of certain environmental contaminants, with the coffin itself being the major contaminant. Another environmental concern is that traditional burial takes up a great deal of space.
https://en.m.wikipedia.org/wiki/CremationFor environmental reasons, natural burial is better than traditional burial.
Natural burial is the interment of the body of a dead person in the soil in a manner that does not inhibit decomposition but allows the body to recycle naturally. It is an alternative to other contemporary Western burial methods and funerary customs.
https://en.m.wikipedia.org/wiki/Natural_burialA good percentage of land area is taken up by cemeteries because of the monuments erected on it.
Metaphorical meaning of headstone.
A grave monument (or headstone) is a sign of respect or fondness, erected with the intention of commemorating and remembering a person.
For environmental reasons (less space) natural burial without headstone is better.
Metaphorical meaning of unmarked grave.
An unmarked grave is one that lacks a marker, headstone, or nameplate indicating that a body is buried there. As a figure of speech, a common meaning of the term "unmarked grave" is consignment to an ignominious end.
The religious aspect connected with burial and cremation is all about afterlife and reincarnation. No room for environmental sentiments.
Google, world running out of burial space and you will realize that it is a real concern in the near future. Burying the dead would turn into a booming business in the future, recycling graveyards.