Nox city
From The Stargate Omnipedia
A vast city on the home planet of the Nox, likely built by the advanced race. Although it floats within site of the Stargate, unwelcome visitors are unaware of its existence, as the Nox use their technology to render it invisible.
KEY EPISODE
The Nox - Anteaus reveals the city to SG-1 to show that the Nox are not as helpless as they appear to be.