The Cyber-tomb was a massive underground complex discovered under the ice and snow of the planet Telos and other tomb worlds. The tomb held a small army of Cybermen and the Cyber-Controller in cryogenic suspension, as well as a cyber-conversion unit, a weapons testing facility and a revitalising unit.
The tomb held what was possibly 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 Cybermen would then awake from their sleep and convert the trapped people into Cybermen. Afterwards, the new Cyber-race would return to Earth and control it. (TV: The Tomb of the Cybermen)
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) 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. They were eventually reawakened by Cybermites. The tombs and their awakened Cybermen were destroyed when the planet was blown up. (TV: Nightmare in Silver) The Twelfth Doctor referred to the Telos ice tombs when name-checking all of his victories against the Cybermen. (TV: The Doctor Falls)