The Vault of Tomorrow, Chamber of the Ages or Vault of the Ages was a location on the planet Janus VI where the Horta placed their eggs for the renewal, a 50,000 yearly self-induced extinction which clearer the planet of it's Horta population for the next generation growing in the eggs inherit.
Prior to the renewal the Horta would lay millions of eggs in the Vault which would then be cared for by a single Horta, the Prime Mother, until they all hatched. (TOS episode: "The Devil in the Dark", DS9 novel: Devil in the Sky)
The Horta underwent a renewal in the 23rd century, in the time when the Horta population was reduced to one the Federation established a mining colony on the planet. The Prime Mother tolerated the Human miners until 2267 when their automated machinery penetrated the Vault and destroyed thousands of eggs. The miners, unaware the objects they were destroying were eggs, continued to mine but the Horta attacked to protect her eggs and over three months killed fifty of the colonists, prompting Starfleet intervention. The USS Enterprise arrived to investigate and after a mind meld between the Horta and the Vulcan Spock an understanding of the Horta race was achieved and the miners learnt of the damage they had inflicted on the eggs in the vault. (TOS episode: "The Devil in the Dark")