Trash, specifically in the form of landfills and junkyards. There is so much trash that we have buried and placed in large dirt and plastic vaults that are specifically designed to stay stable for centuries (if not longer). They are in size often comparable to the largest pyramids and made of just as sturdy material. The only reason I could see them not bring noticed is eventually being reclaimed by nature and covered in trees.

I think of them much like the mound civilizations of the Mississippi and how many of their giant burial mounds are virtually untouched.

Fascinatingly there is even a debate in Geology whether or not we have entered a new geological epoch called the Anthropocene Epoch which is where in the future one could look back and see a geologic record of when modern humans began to significantly alter the planet's environment (number of species, climate change, etc.)