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I've been thinking about how difficult it is to eliminate toxic chemical waste -- it usually needs high temperatures and pressures. So: would it be feasible (and maybe cheaper) to send it into the mantle or core of the world?
I thought about grinding & liquifying our junk and pumping it into a subduction zone off the California coast. But I don't know if that would work; after a million years it might pop up in a California hot springs.

Edit: Instead of pumping junk into a subduction zone, why not just drop the junk onto the sea floor at a subduction zone? It would have to be resistant to ocean water, but other than that it would gradually be pulled below the crust. And presumably be neutralized by the hot high pressure.

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    $\begingroup$ Cheaper? Seriously? $\endgroup$ Nov 1 '20 at 14:05
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    $\begingroup$ Liquid nickel could be pumped out of the core and sold to pay for it. $\endgroup$ Nov 1 '20 at 21:54
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    $\begingroup$ "radioactive waste in subduction zones" $\endgroup$ Nov 2 '20 at 23:47
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    $\begingroup$ Surely you can't be serious??? $\endgroup$ Dec 6 '20 at 6:04
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    $\begingroup$ @EarlGrey: I've heard of underground storage and the like; what I was contesting is the phrase in the headline question 'to near-earth centre'. As far as I understand what we know about the near centre comes from studies of the earths magentic field, of its seismology but not from drilling near the centre! Like I said, you cannot be serious... $\endgroup$ Dec 12 '20 at 4:28
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No. In fact, I don't know why David Hammen didn't say that instead of commenting.

The deepest hole that mankind ever dug was barely a scratch on the Earth's surface when you look at its size. They didn't even reach the mantle. The hole is only 12km deep, took about 20 years to create, and is still only 1/3 of the way to the mantle! Also, the hole is nothing through which you could dump garbage. It's only NINE inches in diameter.

Have you wondered what it means to dump trash into the Earth? Trash takes up space. There are no known voids in the deep Earth. It is very dense. To make room for trash, first you would have to dig the IMMENSELY expensive hole. Doing this also wastes energy. Then, you would need to invent technology to dig out a massive space, working from inside a nine-inch hole that goes over 7 miles into the Earth's surface. How expensive would that be? Is it even possible to carve out a space in the Earth under that much pressure without it collapsing? Given that scientists have only managed to keep open a nine-inch hole, you can bet we currently don't have available technology to build any kind of void for trash, let alone a void big enough to hold enough trash to make more room on the surface.

[Edit: Added 10/25/2021] Just reviewing my comments on this site, I realized I had not mentioned that putting waster quite deep in the Earth may not be viable, but putting waste in a place where the Earth itself will drag the waste away from the surface is an option. In other words, put the waste into a subduction zone. Serious studies have been done on this. Unfortunately, even if the science and engineering for the idea seem sound, people are wary of this method because (a) whoever executes the plan could make mistakes and (b) the plan itself could be flawed, regardless of how many experts designed and vetted it

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  • $\begingroup$ any thoughts on If the Kola Superdeep Borehole were opened today, what would come out of it? $\endgroup$
    – uhoh
    Nov 6 '20 at 2:45
  • $\begingroup$ Also, in creating any sized hole underground, the rock that was excavated needs to be placed somewhere & that is would either be in another hole underground, which could be used for trash ;-), or it would be dumped on the surface. Even if it were possible, it's not a winning solution. $\endgroup$
    – Fred
    Dec 8 '20 at 1:22
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One of the flaws with using subduction zones as dumps for toxic chemicals so they can be broken down by heat and pressure at depth is the rate of sinking of the subducting plate.

Subduction is a very slow process, generally between 2 and 8 centimeters per year. Any toxic material is going to hang around the interface for a very long time, with the potential to contaminate the sea bed and farther beyond, through transportation via submarine currents, for a very long time.

Also, depending on the subduction zone used, the toxic material may resurface relatively quickly, particularly where island arcs are created. This would contaminate the islands made in such environments.

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There's easier ways to mitigate toxic waste, Since most is often metal, converting it into stable oxides and proper landfilling mitigates most of the danger.

Incineration at temperatures above 2800 degrees celsius will totally obliterate most wastes, this is How US disposed of it's chemical warfare stockpile.

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Your idea may seem weird, but it is what has been done for decades in the US and elsewhere.

You do not need to reach the centre of the earth. 3 to 4 km may suffice (Terms and Conditions apply ...)

Have a look for example at the Rocky Mountain Arsenal disposal experience in Colorado USA). In the 60s, the US military had to dispose tons of chemical and batteriological byproducts from their production. You may find details about the site here https://en.wikipedia.org/wiki/Rocky_Mountain_Arsenal but what makes it interesting regarding your question is that the deep disposal of fluids led to ... earthquakes. See relevant publication from USGS archives: https://earthquake.usgs.gov/static/lfs/research/induced/Healy-et-al-1968-Science-(New-York-NY).pdf

Denver area was (and still is) expected to be tectonically quiet, with no earthquakes, and there are convincing proof that the disposal of fluids (toxic, heavy metals, batteriological byproducts, who know what they did dispose) induced some strong earthquakes up there.

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