Cosmic cloud

The cosmic cloud was a moving distortion in space and time that connected two different eras of mondasian history with each other.

One end of the cosmic cloud reached Mondas when it was ruled by silurians and still with its sister planet Earth. The other end reached Mondas when it was ruled by Cybermen and far from Earth.

After a Cyberman scout ship came through the cloud and landed in the Silurians' Mondas, the Silurians sent one of their modified apes through the cloud to discover what was on the other side. This ape crashed in the Cybermen's Mondas and led to the Cyberman becoming curious about the cloud and sending the scout that the Silurians encountered through the cloud. (COMIC: The Prodigal Returns)