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, and are protected by a quantum shield that renders them nearly impervious to damage by locking their structure in place at the subatomic level. They are even capable of surviving 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. Mass relays function by creating a virtually mass-free "corridor" of space-time between each other. This can propel a starship across enormous distances that would take centuries to traverse, even at FTL speeds. Before a vessel can travel, the relay must be given the amount of mass to transit by the ship's pilot before it is moved into the approach corridor. When a relay is activated, it aligns itself with the corresponding relay before propelling the ship across space.There are two kinds of mass relay, primary and secondary. Primary relays can propel a ship thousands of light years but only link to one other relay, its "partner". Secondary relays can link to any other relay over shorter distances, only a few hundred light years. After the Rachni Wars , space faring species won't open a primary relay without knowing where it links to, in case they run into another powerful and hostile species like the rachni. This caused a rift when the turians found human pioneers, ignorant of this Citadel Council prohibition, trying to open any mass relay they could find while exploring the relay network, eventually leading to the First Contact War. Many mass relays are currently dormant for unknown reasons, though they can be easily reactivated. A Prothean data cache found on Mars led humans to a mass relay encased in ice and orbiting Pluto , previously thought to be a moon called Charon, which the Charon Relay was eventually named after. A piece of Prothean artwork depicting a mass relay, the Relay Monument, can be found on the Citadel Presidium. It has been interpreted as either a symbol of Prothean vanity, expressing the relays as their means to build a galaxy-wide empire, or possibly as a symbol of galactic unity, which the relay network also embodies. Tali'Zorah nar Rayya claims that after their long voyage through the galaxy, the quarians have also come to appreciate the aesthetic value of the mass relays. An asari matriarch was once quoted to have suggested creating new Relays, but the potential for current era technology to do so is unknown
The mass relays, despite common belief, were created not by the Protheans but by the Reapers. As stated by Sovereign, the Protheans were merely one of many alien races to find the relays and the Citadel and take advantage of them. When the Reapers wiped out the Protheans, the asari were the next race to find the relays thousands of years later. According to Sovereign, by using the relays, galactic civilizations evolve along the paths the Reapers desire. In addition, the relays serve to accelerate the rate at which those civilizations advance, shortening the time between Reaper harvests.However, the Protheans did have a keen interest in the relays, and managed to crack the secret behind their operation. This enabled them to build the Conduit before they were destroyed by the Reapers. The Shepards also discovers that the Citadel itself is an enormous, inactive mass relay leading to dark space, as well as the control center for all the relays, enabling the Reapers to sever travel between clusters. Its reactivation is, fittingly, more complex than that of an ordinary mass relay, requiring either a coordinated effort by the keepers or manual intervention by a Reaper.