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A device in Akintale created by W. D. Gaster in an attempt to find an alternative reality in which humans either never trapped the monsters underground or never existed at all. This particular version of the RTD was created as a proof of concept as it is obviously far too small for an adult monster to travel through.
The RTD was only activated once. While turned on, it dragged Ace into the Akintale universe, permanently changing the course of history in this universe. Pulling Ace into the world caused the machine to destroy itself with the spinning ring and bulb-like liquid container to shread itself into pieces and explode respectively. WD intended to repair the machine but stopped when overhearing Ace explain just how lucky he was to survive the experiment. The machine was never activated again.
When activated, the machine was in the True Lab. A few decades after Gaster's death, Sans found out about some monsters sneaking into the lab to look at the experiments and steal anything left of value. He retrieved the machine, concerned that someone may attempt to turn it on. After a while, Sans became suspicous of humans having the reset ability but had no real proof. He tried to repair the machine in hopes of contacting Ace after he recieved the first ever genocide attempt nightmare (which happened during Brendon's time in the underground.) After a hundred or so years, Sans gave up on fixing the machine. This is mainly because he didn't have access to spare parts that were required to repair it.
- Apparently travelling using the RTD is a horrifying experience.
- It is unknown exactly what effect it has but those who move through the tears are unconsious after arriving and become panicked apon waking up.
- It is assumed the device shows different things to different people and that they experience different sensations depending on the worlds they travel between.
- It is the only device that allows people to travel outside of the Undertale AU section without travelling through a crossover first.