The Stuff

[anagogy]

when Earth makes the transition to 4th density, what'll happen to all the stuff? i.e roads, cars, buildings, underground/underwater bases, nuclear waste/bombs?
can we contaminate this planet to a point where it is uninhabitable, as those of Maldek?

Unlikely, as Maldek had much greater technology than we have currently, though, I suppose it is technically possible. I don't think planetary destruction would be allowed to happen to another planet in this star system by the federation. I think if it came down to that, they would put their foot down in some way.

But as for all that stuff, it will go the same way that Atlantis went. Atlantis was a highly technological society. In fact, they were further ahead than we are today. Do you see anything left of it? Nadda.

It won't take much time at all for the Earth to swallow up our piddly 3rd density yellow ray artifacts. Nature reclaims land remarkably fast, and our waste, while it may seem significant in our current time-frame is literally nothing from the perspective of geologic time-scales.