Two vast and trunkless legs of stone Stand in the desert. Near them on the sand, Half sunk, a shatter'd visage lies (2-4)
These lines give us several images of nature: the "stone," the "desert," and the "sand." The "stone" reminds us that the statue is a product of nature in some sense; the way in which the legs are standing in the sand suggests something similar, as if they were just emerging from the sand or nature were giving birth to them. "Half-sunk" calls to mind images of the sea: it's as if the head is being reclaimed by an unforgiving ocean of sand. The materials used to make the statue are slowly returning to the place from where they came, completing a kind of natural cycle of life and death.
those passions... which yet survive, stamp'd on these lifeless things (6-7)
"Lifeless" is an incredibly rich word in this passage. That the pieces of the statue are now "lifeless" suggests that they were in fact once alive. Perhaps a work of art is alive when it's complete or, rather, not in fragments like the statue of Ozymandias. Or perhaps it has something to do with the role or function of the work of art in a particular culture. Because the surrounding temples and civilization have been destroyed, the statue no longer functions as a tribute to, or symbol of, Ozymandias's political power; it is "dead" because it is now an artistic curiosity, an object for museum-goers to look at and poets to write about rather than a statue with a specific function within a particular culture.
…lone and level sands stretch far away (14)
Nature has the final victory in this poem: the statue is almost gone, having suffered the same fate as the civilization that produced it. Ozymandias's empire once "stretch[ed] far away," but now it is nature – embodied by the "lone and level sands" – that extends its empire. Interestingly, the sands are "lone" even though there is a statue still there, as if the statue is so insignificant relative to nature that it is almost not worth mentioning.