Ages ago, terrified by a barrage of meteor impacts, they feared that their planet would go through an ice age and they retreated underground in suspended animation. They constructed a sonambutron which would re-activate them again in the next solar eclipse, however they would not predict that in the meantime the planet's moon would be destroyed.
Those Nauians that remained on the surface degenerated into a cat sized insects called Silotis and were the largest native life form on that world.
For the remaining millennia they remained in slumber and their security system would be active harassing the colonists that would settle the planet. These robots would telepathically assume the forms of an intruder's deepest fear.
An away team of the USS Enterprise with Captain James T. Kirk, Spock, Dr. Leonard McCoy and Ensign Everts arrived on the planet to investigate. They discovered the doorway into the sleeping complex and managed to reactivate the machinery which awakened one of the Nauians who explained his species plight.
As a reward for their assistance, the Nauians agreed to share their worlds with the settlers and also wanted entry for his people into the Federation as they believed they had much advanced technology to share. Captain James T. Kirk agreed to make an application for membership for the race and departed the world allowing the Nauians to awaken on their world once more. (TOS video game: Star Trek: 25th Anniversary)