Sentence:  But knowest thou that trees and fruits flourish upon the earth / To gratify senses unknown—trees, beasts, and birds unknown; / Unknown, not unperceiv'd, spread in the infinite microscope, / In places yet <WSD>unvisited</WSD> by the voyager, and in worlds / Over another kind of seas, and in atmospheres unknown? 
Meaning of the word marked with <WSD> tags:  Not visited.