Also "modern" landfills when filled tend to be "capped" (covered in plastic) so rain doesn't leach though into the groud water. If they have a bottom liner, these landfills are like the worlds largest plastic bags.

To prevent methane building inside the capped landfill, pipes are run and the gas is collected and sometimes "flared" [1]

Having dug into some old old landfills, it was like a time machine of 25+ year old stuff. nothing seems to rot well inside them (no water). papers readable. We found a glass bottle of what appeared to be grape soda.

[1]https://www.epa.gov/lmop/basic-information-about-landfill-ga...