Landfill decomposition
Landfills have special liners (like swimming pools) made of plastic and clay to trap liquids. These liners are at the bottom of the landfill to keep nasty liquids from seeping through and ruining nearby water wells or other bodies of water such as lakes and rivers.
When the landfill is full, it's covered with dirt, tightly packed. That's why even food garbage doesn't decompose quickly in landfills. Air, water, heat, and microbes are needed for decay. Now you understand why you can sometimes still read a newspaper found in a landfill after 20 years!
