Stasis cube

A stasis cube was a form of Time Lord art found on Gallifrey, which the Eleventh Doctor described as a "little sliver of time" held in perfect stasis in a frame. The result, according to Clara Oswald, resembled a 3D oil painting.

In the early 21st century, several stasis cube paintings, including one titled Gallifrey Falls No More, were in the collection of dangerous art housed in the Under-Gallery of the National Gallery in London, where figures disappeared from the pictures and the glass over the cubes shattered. It was soon revealed that the figures were Zygons, who had used the stasis cubes to freeze themselves in 1562 and wait until the planet was more technologically advanced. This led to a meeting between multiple incarnations of the Doctor — the Tenth Doctor, the Eleventh Doctor, and the incarnation who fought in the Time War.

Using the method of implanting themselves in stasis cube paintings the Zygons had been using, the Doctors froze themselves in the Gallifrey Falls No More painting in order to access UNIT's Black Archive in the Tower of London, which was TARDIS-proof, and avert the destruction of London by self-destruct during a showdown between the Zygons and Kate Stewart. (TV: The Day of the Doctor)