Cyber-tombs, also known as ice tombs, (TV: Earthshock, The Doctor Falls) were massive underground complexes which held armies of Cybermen in suspended animation. Cyber-tombs were discovered on Telos, Hedgewick's World, and other tomb worlds. They sometimes contained cyber-conversion units, weapons testing facilities, and revitalising units.
The tomb on Telos held what was believed to have been the last of the Cybermen, keeping them alive so the Cyber-race would live on. It was designed to be a trap. If discovered at sometime by humans (which must possess great intelligence in order to solve the Cybermen's symbolic logic control systems), as soon as anybody walked into the tomb, it would seal itself closed, trapping the people inside. The Telosian Cybermen would then awake from their sleep and convert the trapped people into Cybermen to continue the species. Afterwards, the new Cyber-race would return to Earth and control it.
The Second Doctor visited this tomb when Eric Klieg was trying to revive the Cybermen. Although Klieg succeeded in awakening the the Cybermen, they were soon sent back into hibernation and the tomb was sealed once more. (TV: The Tomb of the Cybermen)
Hedgewick's World of Wonders
The planet where Hedgewick's World of Wonders was built had a Cyber-tomb underneath the theme park, which housed three million Cybermen in stasis, recovering from the effects of the Cyber Wars of the 250th Millennium. By the time the Eleventh Doctor arrived with Clara Oswald, Angie and Artie Maitland, the theme park had been closed due to visitors being abducted to repair the damaged Cybermen.
After the Doctor's arrival, the Cybermen were unleashed while the Cyber-Planner Mr Clever attempted to take over the Doctor. Outnumbered, Emperor Ludens Nimrod Kendrick Cord Longstaff XLI, who had been hiding out on the planet, reluctantly activated the Desolator, a planet-destroying bomb which obliterated the planet, the Cybermen and their tombs. The few survivors of the battle were rescued by the Imperial Flagship, summoned by the bomb's activation and the ship scanned the debris for life, finding no signs of any Cybermen survivors. However, it missed a single Cybermite which remained active in the debris field. (TV: Nightmare in Silver)
It was later discovered that multiple tombs existed across the galaxy, including ones converted from human colonies. (AUDIO: The Crystal of Cantus) Cyber-tombs were seen among the ruins of a Cyber-ship that crashed into the Arctic Circle. (GAME: Blood of the Cybermen)