I predict that within the next 20 years, closed up landfill
s will be mined
for natural resources
Let's think about this: What's in a landfill? Well, lots of things that aren't worth digging up, most likely, like most of the organic matter in there like paper, wood, rubber, etc. However, since most of the stuff in landfills was put there before recycling, there's also tons of things like glass, aluminum, and steel, as well as plastics that were not recyclable 20 years ago but can be now. Discarded electronics means there's copious amounts of copper, silver, and gold. Most mining for these metals in particular is occuring in deposits with very low natural concentrations (most of the high-yield ores have long since been removed) so it's possible your average landfill actually has a higher copper content than an actual copper mine.
There would be a few problems with landfill mining. There's a lot of toxic stuff in most landfills--everything from cleaning supplies to industrial waste and toxic heavy metals. Extracting and refining the materials would be difficult, and is probably the biggest reason why it's not being done now. Of course, traditional mining is also incredibly toxic (most gold mining at this point uses cyanide to extract gold), so comparatively it may not be so bad.
I think, though, as natural reserves of raw materials dry up and exploration and mining costs increase, and technology improves, landfill mining may become an economical alternative or supplement to conventional mining.