10 Strangest Worlds In Star Trek
9. Meridian
Meridian, or Trialus 1, was a planet in the Gamma Quadrant that existed in this dimension for a short period of time. In fact, when the USS Defiant entered the Trialus system, the planet popped up to say hello - for the first time in sixty years, and even then it was only due to stick around for twelve days.
The locals weren't sure what caused this dimensional hopping, though knew enough to say it was related to the Trialus star. Between periods in normal space time, the planet and everything on it shifted into a realm of pure consciousness. This was wonderful as an anti-aging method but the shortened stays in normal space had effectively killed the chance for reproduction (the old-fashioned way).
Despite Jadzia Dax's affections for a local named Deral, she could only pass on the knowledge of how to help, rather than join Meridian on its next hop into consciousness. Though she believed she had altered her body chemistry enough to join them, it wasn't to be, and her presence almost destroyed this world and everyone on it.
Jadzia herself wouldn't be around for Meridian's next phase into this reality, but it is entirely possible that Ezri Dax could have popped along to say hello in about 2431.