Originally an important Asgard world, Halla was designated by the High Council to be the planet where a massive experiment would take place. It is presumably located in the Asgard home galaxy of Othalla.
Preparing a trap with a time dilation device, the Asgard initiated a command to bring forth all Replicators in the galaxy to the planet, hoping to trap them there until a way to deal with them was devised. However, the Replicators were able to alter the device and reverse it's effect.
The planet became home to every known Replicator in the galaxy and the planet's surface was terraformed by the machines, with virtually every inch of its surface covered with layers of Replicator blocks.
Eventually the Asgard give up on finding a solution to the Replicators. They turn the planet's sun into a black hole and the planet is pulled into it.
HOME TO: Destroyed
FIRST APPEARED: Unnatural Selection
Unnatural Selection - At Thor's request, SG-1 takes the Prometheus to Halla to activate the Asgard's time dilation device in the hope of trapping the Replicators. They find that the world has been completely transformed with Replicator blocks.
New Order, Part 1 - The Asgard turn the Halla system's sun into a black hole to destroy the Replicators but a group of them, including Fifth, find a way to escape.