Radiation, a billion tons of plastic, dioxin, PCBs, CO2 at record levels, ozone-depleting chemicals - these are the legacy we would leave behind if humans were to vanish tomorrow. The giant structures would mostly be gone or unrecognizable in decades or centuries, dissolved by water and oxygen, covered with plant-life. But our chemical and nuclear creations will continue to be a plague on the planet for a very long time.
Weisman describes in some detail the effects of the Chernobyl melt-down. The whole of Europe contaminated to varying degrees, and a death toll that will probably never be accounted because so many of the deaths will be years after the event from cancers started by the radiation. Now, imagine 441 Chernobyls and you have some idea of what would happen if humans no longer maintained the existing nuclear reactors. Plutonium 239 in the reactor cores would decay to become Uranium 238, with a half-life of 4 billion years, and would be expelled into the atmosphere when the containment vaults are breached by high temperatures - truly the gift that keeps on giving.
Plastics do not biodegrade - no organisms have yet evolved to break down plastic into its constituent hydrocarbons. So the billion tons of plastic produced since the 1940s is all still in the environment. It doesn’t biodegrade but it mechanically degrades into smaller and smaller particles - thus providing a choking and suffocation hazard to more and more smaller and smaller creatures.
The PCBs that were used for softening plastics until the late 1970s when their carcinogenic properties became publicly known (long known before that to the plastics industry, of course) will remain in the environment essentially forever. Only UV can destroy PCBs, so until PCB molecules rise into the upper atmosphere, they will remain intact. Another gift to species wishing to mutate at a faster than normal rate, I suppose.
These effects are largely due to essentially criminal negligence on the part of corporate capitalism, of course, but it is also true that to really reverse the damage that we do it will be necessary for the world human population to revert to levels not seen since the early 19th century. This can only happen if women do not have more than 1 child. Multiple children was a survival tactic when humans were primarily farmers or hunter-gatherers and when infant survival rates were very low. But that is not our situation. Now we are faced with environmental disaster and resource constraints. Engineering and socially beneficial politics can go only so far - we need to convince or coerce people worldwide to stop having so many babies.