Sentence:  Falling somewhere in a category between Einstein 's theory and sand fleas - difficult to <WSD>see</WSD> but undeniably there , nevertheless - is the tropical green `` city '' of Islandia , a string of offshore islands that has almost no residents , limited access and an unlimited future . 
Meaning of the word marked with <WSD> tags:  perceive by sight or have the power to perceive by sight