10 Biggest Unanswered Star Trek Mysteries

3. The Aliens From Schisms

Easily one of Star Trek's all-time most underrated Episodes, Schisms sees several members of the Enterprise's crew abducted from the ship by a still-as-yet unidentified alien race. While gone they're subjected to all manner of grizzly examinations, resulting in the death of Lieutenant Hagler.

While Commander Riker is able to escape the pocket universe and the crew able to close the rift that's enabling the abductions, virtually nothing is learned about their captors or their motives. An energy pulse escapes the ship, flies off into the galaxy, and is never seen or heard from again.

Picard: Have we any idea what came through the rupture before we were able to shut it down? Data: No sir. We were unable to track it once it left the cargo bay. LaForge: Maybe it was a probe of some kind. Data: Possibly they were simply curious. Explorers like ourselves. Riker: Ensign Rager and I were lucky to have escaped. Lieutenant Hagler's dead. Whoever it was sent that thing was more than simply curious.

Neither The Next Generation nor DS9 or Voyager, have ever gone back to the well on this one, meaning the origins of the strange, amphibian-looking aliens have yet to be learned.