Mass relays are built with two long, curved metal "arms" with a set of revolving rings in between them, containing a massive, blue-glowing element zero core. The relays are made of an unknown but incredibly resilient material, the same material that the Citadel is built from. One of the relays even survived a supernova's wake without being damaged. They are "cold" objects that don't emit heat or radiation, unlike starships, making them difficult to find if their position changes. Some relays, like the Charon Relay, are "gravitationally anchored" to planets; others appear to be out in space and are carefully tracked.