Prison Asteroid K-37 is Dr. Hämsterviel's "prison cell" seen in the Lilo & Stitch franchise. He has set it up to be a laboratory, complete with teleportation systems, and can be revealed or hidden using an encrypted voice code. It is also where Hämsterviel keeps a number of captive experiments hidden away in a secret cache, which Gantu had managed to capture and teleport to him.
In the series episode "Woops," Hämsterviel activates Experiment 600 (Woops) that was transported to him by Gantu. However, Woops turned out to be a real klutz, and after causing mayhem around the prison, freeing Nosy (one of the captive experiments), and alerting the prison guards, Hämsterviel was forced to teleport Woops and the other captive experiments back to Earth on Gantu's ship, with orders to get rid of Woops.
In Leroy & Stitch, Gantu breaks Hämsterviel out of prison. Hämsterviel then pays Jumba Jookiba visit at his lab and forces him to create a new version of Experiment 626 (Stitch). When Jumba finishes doing so, Hämsterviel christens the newly created experiment Leroy. After Leroy defeats Stitch, Hämsterviel clones Leroy into an army to overthrow the Galactic Alliance. However, in the end, Hämsterviel, Leroy and his clone army are defeated and taken back to Prison Asteroid K-37, where they are all put in individual prison cells.