system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If we get uranium from the earth, why is nuclear waste such a big deal?

If uranium exists naturally in soil and ocean water, why is there such a big deal about environmental contamination with nuclear waste? Why can't we just put it back when we're done with it, leaving the mining site as it was before? If it came from the environment, why do we try so hard to keep it away?
gpt
Because nuclear waste is not just leftover uranium. Regular uranium is fairly stable and harmless, but the waste after splitting it is not. It is highly radioactive, damaging to humans and all other life. 

Just like oil also comes from the ground, but when burned to CO2 it's a harmful greenhouse gas.