What would cause them to be an environmental issue? As far as I know they can be built in a safe manner and there is plenty of space for them.

Most landfills contain icky things like heavy metals or arsenic. Having that leak into an aquifer Would Be Bad.

That’s why they’re sealed with clay and plastic and constantly monitored for leakage.

If it needs constant monitoring it's a potential problem... And at some point it will happen... We know that from ages of software development, if it can fail, it will fail at some point.

You’d be surprised by the number of things that need to be constantly monitored. Building structures, power plants, roads, dams, mass transit, etc. basically everything.

Dams break and wreck havoc on ecosystems, the US has had it's fair share of coal mine sludge disasters. Even countries with strict environmental agencies and long history of mining have accidents.

So what's your solution? Stop building dams and decommission the ones that have been built? Humans attempt to create order. The universe naturally wants to increase disorder. It's natural that anything we create will need to be monitored.

We know that dams are dangerous and that they fail, the smaller ones are decomissioned when not in use not only to improve the ecosystem. A dam is not designed for a life of a 100 thousands years, it's a long time.

In the Western world maybe, but outside of it... hell even in the Western world we have regular reports of mismanagement and corruption, just look at Italy.

We need to reduce the amount of trash we produce, not hope for a miracle technology.

Landfills are not a miracle technology. They exist already and are technologically sound. It’d be way more of a miracle if we somehow managed to greatly reduce waste.

People love to "what if" landfills. Best to not bother arguing with them. We have tons of problems with emissions, plastics pollution, industrial dumping. However landfills ain't one of them, but people won't give up. I've learned to not bother discussing anything with them if they don't know a few basics of how modern landfills are run (at least in western nations).

As others have mentioned they are a major source of methane emission, though this can be addressed (do a better job separating out food waste and compost it).

Landfills were demonized for a long time because they were easy targets.

It's usually just disguised NIMBY-ism :)

The thing about nimbyism in this case is that confortable commentators on hackernews are vanishingly unlikely ever to have to live near one. This is a poor people problem.

There’s a famous landfill right in Silicon Valley though; it’s caused Milpitas to smell like rotten eggs for the last 20 years.

https://sf.curbed.com/2019/3/19/18273029/milpitas-odor-studi...

Correct, landfills are not a miracle technology. Landfills leak.

https://www.clf.org/blog/all-landfills-leak-and-our-health-a...

https://pubs.usgs.gov/fs/fs-040-03/

How sustainable are landfills? There's limited land for farming, residences, manufacturing, and everything else we use it for. And trucking trash to deserts to landfill there isn't carbon neutral either. Trash also doesn't breakdown as quickly in covered landfills (for better or worse depending on what one dumps).

Given the yield of useful content vs. rock in mining, I suspect the more limiting factor is space for all the mining, not space for disposal of the 0.1% metal etc yield for making industrial goods.

If you are worried about space i recommend that you look at the surface area of earth and the volume of space that’s accessible to us underneath the surface.

Is there enough raw material on earth for landfills to become major environmental problems? (genuinely curious)

Depends what you mean by "major environmental problem". Landfills aren't going to singlehandedly cause anthropogenic climate change, but they contribute significantly to it, and a poorly sealed landfill leaching its contents into the local water table can cause some pretty drastic problems with the local environment

How are they worse than any alternative? If I dump my food waste in the garden, is that emitting less GHG than letting it decompose in a landfill?

Is it that it comes out as methane from landfills but less harmful CO2 from a little pile? That should be solved by flaring the methane.

Maybe this makes me sounds like a jerk, but:

All material that can end up in landfills is already on Earth.