Xel'naga caverns

The Aiur caverns are a giant underground system found beneath the surface of Aiur. It is an artificial city constructed by the Xel'Naga.

There are at least two ways into the caverns, a large obvious passage and a secret "back" passage.

The caverns are decorated in many ways, including ropy "swirls" which resemble vines. During the Aeon of Strife, a "hearbeat" sound echoed in the caverns, getting louder as one approached the large crystal chamber.

The system is very large, and one can be easily lost within it. It consists of tunnels connecting nearly-identical chambers.

Back Passageway
The back passage normally appears as a flat, bare area with hundreds of Khaydarin crystals. Entering the Golden Mean pattern into the crystals causes a giant block of earth to rise, revealing a stairway.

First Chamber
The first chamber at the base of the stairway has five tunnels leading from it.

It features a gem-studded pillar. Activating the gems (using the Golden Mean pattern) causes a platform to extend from the far wall. During the Aeon of Strife, the platform held dessicated Protoss "corpses" taken from different tribes. These corpses were attached to the vine system.

Some time after the Brood War, the chamber held the corpses of the Ulrezaj.

Giant Crystal Chamber
Deep within the caverns lies a chamber with a giant floating extremely pure Khaydarin crystal. It seems to house an artificial intelligence inside.

The room houses empty platforms like those seen in the first chamber. It was here that Savassan learned many secrets of the Xel'Naga by attaching himself to the vines. He was drained by the vines, then restored. During this time, he shared information with the giant crystal. This was a pivotal moment in Protoss history, since it led directly to the end of the Aeon of Strife.

When the Tal'darim cult took over the caverns, their Benefactor, the Xava'tor, forbade them to enter the chamber on pain of death.

Large vats containing strange psionic creatures were put into the room. Their psionic emanations terrified the Protoss and psychic Terran who encountered them. The creatures were only glimpsed, and their true nature remains unknown.