system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If we extract oil from underground "reservoirs", why don't we dump toxic waste in them after they've been emptied?
gpt
You've removed their seal. To access those reservoirs you had to drill a hole to them, which placed new stresses on the surrounding rocks and perhaps relieved others. 

Then you pumped the oil out, which removed a lot of the "load bearing" liquid (much like the water in a water bed) that held up the rock above them. 

So the conditions have changed for that now-empty reservoir, even if you completely cap and plug the hole you used to bore into it - and that can create a risk if you use it for toxic waste storage.

As a result, some reservoirs would be unsafe for toxic waste disposal, although others might be very stable and a good place to dump it... but you still have to ship it there and get it down the small hole. And if the technology for oil extraction advances (as it has in the past bunch of decades), you can no longer access any of the oil in that hole, or any oil that might be nearby it due to the risk of contamination.