What are Necron Tombs?

[Todokoro]

I keep seeing segments all over the map with glowing green cracks all over the ground. when my mouse hovers over said segment, it just says there is a Necron Tomb there. When I searched for it Google kept barfing out bloated info on the necrons rather than what these things are or how to exploit them. Or are those faction dependent?

[Bullitmagnit]

Necron players can build cities there.

[Corseth]

Only Necrons can make use of them, and those are the only tiles they can build cities. They have to move a tomb spider to them, so it won't just pop up spontaneously, and they don't have any other effect so other factions can ignore them, except for knowing that's where a Necron player has to go to expand.