Made the mistake of clicking on "Explore" in Photos (v3) on Mac OS, how do I hide "Related" panel?????

I was looking for a better way than the thumbnails at the bottom to scroll through Photos, so I made the mistake of clicking on "Explore".

Now I can't figure out how to re-hide the annoying and unnecessary "Related" panel that opened across the bottom of the main Photos pane, above the thumbnails. Definitely what I did NOT intend to do!

Cannot find anything about this rotten Easter egg in the help material.