Meaning of "has still never seen"

I found the following sentence form a book. What I cannot understand is the subject of the bold phrase. Therefore, I would really appreciate it if someone could paraphrase the bold phrase to understand it properly. In fact, I can understand the meaning of the sentence without difficulty, but the thing is I cannot understand the associated grammar points.

Robots crawl on the surface of Mars, and spacecraft exit our solar system, but most of our own planet has still never seen by human eyes.