Upper Kingdom

The Upper Kingdom is one of the sections of Azjol-Nerub.

Warcraft III
The Upper Kingdom was the area seen during the mission "Ascent to the Upper Kingdom." It was seemingly only seen in that mission. It encompassed the sections of Azjol-Nerub stretching from the area beyond the fight with the Forgotten One to the Azjol-Nerub entrance to Icecrown.

When Arthas entered the Upper Kingdom, he faced collapses caused by unexpected quakes and had to get beyond various nerubian traps while Anub'arak managed to get through by tunneling.

Description
The Upper Kingdom is one of the two sections of Azjol-Nerub, the other being the Old Kingdom.

This area includes more recent structures, built when the nerubians grew too numerous to stay below. The War of the Spider was fought through here, so much of this area is destroyed. The Scourge now occupies this level; they've reopened damaged passages and cleared rubble-filled tunnels, but they did not bother to repair the many statues, friezes and murals that were ruined.

Battle.net map
Here in the shattered kingdom of Azjol-Nerub, pillars of ice pierce the land like the teeth of an enormous beast. Perhaps recruiting an ally from the native inhabitants would be wise. Note that this map was obviously above ground, whatever that may imply.

Wrath of the Lich King
refers to the section of Azjol-Nerub that Anub'Arak is fought in as the "Upper City." This may refer to that section of the city being part of the Upper Kingdom.

Azjol-Nerub was later confirmed to take place in the Upper Kingdom.

Judging by its location, Naz'anak: The Forgotten Depths is probably a part of the Upper Kingdom.