Zeno's paralock

s were an invention of a Time Lord named Zeno that he considered to be the most secure locking mechanism in the universe. It worked by putting aeons of time in front of the door, making it impossible to reach it during a person's lifetime, unless they had a key.

It was rumoured that Zeno built three examples. One Zeno's paralock was installed in the Galactic Seed Vault to lock the door leading to the Genesis Seed. It required three keys to be opened, one of which was the TARDIS key. The Thirteenth Doctor estimated its time barrier to consist of about sixteen billion years.

The Doctor did not believe Zeno's paralocks to be impenetrable. She conjectured that the noughtweed modified with a time propagator would only take 48 hours to cut through the paralock. (PROSE: The Secret in Vault 13)

Behind the scenes
Zeno's paralocks are based on real-world by. Like the paralock, the real-world paradoxes, especially the dichotomy paradox, purport that it is impossible to cross a distance from one point to another. While the paralock prevents it by putting vast (but finite) amounts of time as a buffer, the dichotomy paradox purports that there is always a buffer consisting of infinitely many actions, namely actions of reaching half of the distance to the other point, then half of the remaining distance, then half of then remaining distance, and so on ad infinitum.