We already mine landfills -- mostly for land reclamation but sometimes to recover resources.
replyIn the longer run, when there's been more compaction, settling, and densification (and changes in what things are valuable), and more need to reclaim land that was previously landfilled, we will do this more.
People sometimes build stuff on top of landfills.
replyExample: Shoreline Amphitheater, near Google HQ in Mountain View. Built on top of a landfill. For a while in the 80s, there were occasionally small fires during shows when people lit cigarettes. Google also harvested the methane and used it to power some stuff, although I can't find an authoritative article with details.
replyIndeed, sometimes big things. The landfill we used when I was growing up is now beneath a Home Depot, which was built over the top of it almost 25 years ago. The landfill in this case was unlined, too.
replyYup. It is a little undesirable for various reasons, and not every landfill is suitable for construction on top (seismics, sealing/capping technique, materials, etc).
replyLike ski courses!
replyToday’s landfills are already used for natural gas generation.
replyA british inventor created a setup with two long vibrating plates with ferrofluid in between. A flaky powder made from garbage was dumped in on one side and came out the other end beautifully separated in many layers by density. (with one mixed layers in between that went back in at the beginning) Innitially he "knew" it was silly to use something as expensive as ferrofluid but planned to try other substances if it worked. It turned out the process produced a lot more ferrofluid than it used.
replyNo one was interested in further research.
edit: I see some research is now happening.