Tanis is located in the Great Wastelands. It contains the great derelict of Tanis.
In biblical history, Tanis was the home of the Well of the Souls, the last known resting place of the Ark of the Covenant. Homeworld is known for its biblical references (most notable among them Kadesh, the site where some of Moses' people abandoned him to settle). One can only guess at what metaphor is in use here.
If you look at the following image, it shows Tanis without the extreme lighting and shaders that the level presents.http://images.wikia.com/homeworld/images/8/87/Tanis_shipyard_side.jpg
As you can see, minus the obvious tampering of the Hiigarans, it shows the same hull hue and saturation that is common with other progenitor derelicts, megaliths, and ships found around the galaxy.
In the age of the Progenitors this probably also served as a Super-fortress/Shipyard, Tanis may of had a part in the construction of the Progenitor Mothership as well as several other ships. Which would be fitting seeing as the Hiigarans used it for a similar purpose.
I personally believe that it exists in it's current state (Because it is not a part of the Karos Graveyard.), because it was a battleground of the progenitor factions Abassid and Bethsidea. (Although, that's if you believe that internal conflict was the reason that the Progenitors disappeared.)