'Agents of SHIELD' Just Revisited a First Season Cliffhanger in a Major Way

Tonight's Agents of SHIELD saw the return of an important element from the show's first [...]

Tonight's Agents of SHIELD saw the return of an important element from the show's first season.

SPOILERS for Agents of SHIELD Season Five Episode Seven, "Together or Not at All," follow.

The SHIELD team is currently stuck decades into Earth's future. Something destroyed the planet, and what remains of humanity lives on a space station under control of the Kree.

"Together or Not at All" followed up on Fitz's rescue of Daisy and Simmons. As the three of them ran through the Lighthouse, they ended up in a life support center. Fitz took a look around the room and discovered several canisters of gravitonium. This seems to be how the Lighthouse maintains its state of artificial gravity.

Gravitonium is a rare element with unique gravitational properties. The element first appeared in the Marvel Cinematic Universe in SHIELD's first season.

The episode "The Asset" introduced both gravitonium and the character Franklin Hall. Hall was a scientist and SHIELD asset. He worked on many theories about gravitonium even before confirming its existence. As such, he may have been the only person in the world capable of manipulating it. He designed a machine that could stimulate gravitonium. In theory, this would allow him to use its unique properties for various purposes.

It was the villain Ian Quinn who discovered the first large gravitonium mass. Hall arranged to have himself kidnapped by Quinn so that he could sabotage the project.

Coulson launched a mission to rescue Hall. During the mission, Coulson discovered Hall's plan, and he also realized it would do immense damage to the local population of Malta. Coulson stopped the plan from moving forward, but the gravitational effect drew Hall into the gravitonium mass. Though no one knew it, Hall was still alive within the mass.

Coulson sent the gravitonium to a SHIELD storage center called the Fridge. It was later taken by Raina and John Garrett. When Garrett gained the powers of the super-soldier serum, he remarked that he could see the mass's soul. This seemed to hint at things to come from Hall, but Quinn and Raina left with gravitonium. It hasn't appeared since until now.

Many fans were anticipating Hall's reemergence as his comic book supervillain alter ego. As Graviton, Hall as the ability to manipulate gravity at will. So far, the Avengers villain hasn't appeared in the MCU. Could this be a hint of things to come?

Marvel's Agents of SHIELD airs Fridays at 9 p.m. ET on ABC.