The latest episode of Agents of S.H.I.E.L.D., “Principia,” has reintroduced an old S.H.I.E.L.D. concept in a big way. Gravitonium is back in play, and it may be the secret to saving the world in this corner of the Marvel Cinematic Universe.

The extremely rare element was introduced back in Season 1’s “The Asset.” Canadian scientist Franklin Hall first theorized the existence of Gravitonium 20 years ago, and even suggested that the element could have unusual gravitic properties. Inspired by his classmate’s theory, Ian Quinn dedicated a great amount of time and effort to discovering Gravitonium. He was eventually successful, proving that the scientific theory was - as far as the MCU is concerned - scientific fact.

Gravitonium joins the list of the MCU’s rarest elements, alongside Vibranium. So just what Gravitonium is out there, and where is it?

Ian Quinn’s Gravitonium Mine

It’s important to remember that Gravitonium is actually a naturally occurring element, although it’s extremely rare. In “The Asset,” Quinn admits that he searched for 20 years, across six continent and 12 major mines, before finally discovering a deposit of Gravitonium. This deposit is, to date, the only known source of Gravitonium in the world. That raises the possibility that, like Vibranium, the element actually has some sort of extraterrestrial origin.

Curiously, Quinn nowhere indicated that the Gravitonium mine had been tapped out. Quinn’s assets were frozen when he was finally arrested by S.H.I.E.L.D., and it’s unknown how this affected Quinn Worldwide; at the very least, it may mean the Gravitonium mining operation ground to a halt. That means there’s potentially a Gravitonium mine out there, and only Ian Quinn and some of his key shareholders would be able to lead S.H.I.E.L.D. to it.

Cybertek’s Gravitonium

The largest sample of Gravitonium seen to date is the piece acquired by Quinn’s mining operation. This large mass of Gravitonium was taken to Malta, where Quinn experimented upon it in order to harness its gravitic properties. Dr. Hall attempted to sabotage Quinn’s experiments, almost causing a disaster, but S.H.I.E.L.D. saved the day. They promptly confiscated the Gravitonium, transporting it to the Fridge for safekeeping. Unfortunately, shortly after Hydra reared their head; when Ward and John Garrett raided the Fridge, they stole the Gravitonium. It was passed back to Quinn, who seems to have continued working with it in an alliance with Cybertek. “Principia” suggested that General Hale is now in possession of the majority of this piece of Gravitonium.

This is probably the most important sample of Gravitonium, not least because Dr. Hall was actually absorbed into it. Season 1 repeatedly teased that Hall is still alive, and that’s a plot thread Agents of S.H.I.E.L.D. is sure to revisit at some point.

Meanwhile, the remainder of the Gravitonium the former Cybertek scientists were transporting - before the ship was struck by lightning and the majority was taken by Hale - is now in the possession of S.H.I.E.L.D. after “Principia.”

Deke’s Gravitonium Sample

Gravitonium has been a far more frequent sight in Season 5; the time loop seems to include S.H.I.E.L.D. gathering all known samples of Gravitonium to the Lighthouse, where they are used to generate an artificial gravity field. Deke carried a smaller sample on his belt, allowing him to nullify the Lighthouse’s gravitational field. He brought this sample back in time with him, and Fitz has used it as a temporary seal for the dimensional rift.

So far, these are the only samples of Gravitonium known to be in play. It’s understandable why S.H.I.E.L.D. face a challenge to track more Gravitonium down; the only known mine is a closely guarded secret, one held by a fugitive who has evaded justice since Hydra revealed themselves. If S.H.I.E.L.D. really are going to acquire more Gravitonium, the only possible solution is confronting General Hale and her dangerous daughter, Ruby.

MORE: Agents of SHIELD Should End With Director Daisy Johnson

Agents of S.H.I.E.L.D. continues Friday March 23rd with “The Devil Complex” at 9pm on ABC.