The Hall of Mjollnir is the name given to the passageway within the labyrinth deep underground on Cimmeria. As a means of protecting the planet, a monolith near the stargate uses a scanning beam to identify the Goa'uld, known as Etins, and sends them to the labyrinth. There a holographic projection of Thor explains that the labyrinth leads to Thor's Hammer, a device from which only the host can leave alive.
Cross Reference: Asgard, Asgard Transport Beam, Cimmeria, Cimmerians, Hall of Thor's Might, Hall of Wisdom, Holographic Projection, Kendra, Thor, Thor's Hammer
Episode Reference: Thor's Hammer