It depends entirely on the content of the waste and the levels of moisture and air infiltration. Some landfills are known to generate huge amounts of methane,

It depends entirely on the content of the waste and the levels of moisture and air infiltration. Some landfills are known to generate huge amounts of methane, and ooze black sludge at the surface. This famously occured after the 1989 San Francisco earthquake. In contrast, landfills in arid places can preserve trash like a museum: https://www.latimes.com/archives/la-xpm-1993-07-17-mn-14066-story.html https://www.latimes.com/archives/la-xpm-1993-07-17-mn-14066-...