Ortus Mallum (loosely translated as "Bad Place"; a more literal translation would probably be "Place Arisen of Evils") is the village that served as the original home of the Alterans.
The village is located on a mountain on the planet of Celestis, near the Plains of Celestis. It is the place where the Ark of Truth was created. When the Alterans left Ortus Mallum for parts unknown, they did so in a ship that burst out from within the mountain, causing it to partially collapse and bury the village. It was here where the Ark remained until SG-1 arrived on the planet and Dr. Daniel Jackson, Vala Mal Doran, Tomin and Teal'c discovered it within the buried ruins of the village.
Today, the village has been replaced by a forest. After they had found the Ark, they had to move it to the surface as the underground ruins were unstable. The Odyssey had to jump to hyperspace seconds before, due to four Ori warships arriving nearby and thus it was on the surface where they were ambushed and captured by the Ori army. Daniel and Vala were taken to the City of Celestis while Teal'c was left behind and believed to be dead. (SG1: "Origin", "The Ark of Truth")