It may have been per year, I don’t recall. The waste in that photo does not seem very deep, so it’s possibly taking up a lot more area than needed.

So if we go by that stat (300 million tons per year) we need 0.3km^3 per year for the United States to store all of its waste. So let’s say we have a hundred years worth, that’s 30km^3. Could we dig a hole 30km^3 somewhere? Not really, we can’t dig that deep generally. If we made this hole (or structure, it could have walls and a net over possibly) 100km by 100km it would need to be 2.7km deep/high to store 100 years of garbage.

It would be difficult to do this, and would probably need more area to be feasible, but I would think it would be less of a mess environmentally to try to contain the problem to a few sites like this rather than having smaller poorly contained dumps all over the world.