WARNING: The following contains spoilers for Eternals, in theaters now.
A new Marvel Cinematic Universe fan theory suggests there may be a connection between the earthquake mentioned in Avengers: Endgame and the Emergence of the Celestial Tiamut, who was eventually defeated in Eternals.
Trailers for Eternals showed group leader Ajak (Salma Hayek) explaining the Blip, and then the subsequent reappearance of half of the population, triggered "the Emergence" -- something Ajak and the other Eternals now only have seven days to stop. Details of what the Emergence entailed weren't included in the trailer, leaving fans to speculate about it.
Through a series of revelations in the film, the audience learns the Emergence refers specifically to the birth of a new Celestial, the MCU's first known beings. The Celestials were introduced in detail in Guardians of the Galaxy, Vol. 2 with Ego (Kurt Russell), Peter Quill's biological father, who displayed the species' great and overwhelming power as well as their ruthless nature.
As Arishem (David Kaye) explains, Celestials seed the planets they create with new Celestials. Eternals are sent by the Celestials to help the planet's sentient population develop and grow, while Deviants attack and weed out the planet's predators. Once the population is large enough, the Celestial will emerge and take form, destroying the planet in the process. Eternals follows Sersi and the others trying to stop this Emergence after learning the truth because they don't want to let the population of Earth die.
When an Emergence is on the way, the world will show signs, including strong earthquakes, as shown in an early Eternals scene during which Sersi and some of the children she's teaching live through a massive quake. It's not entirely clear how long before an Emergence that the earthquakes begin, only that it's part of the lead-up.
During an early scene in Endgame, some of the remaining heroes, led by Natasha (Scarlett Johansson), are having a conference call and discussing current events both on Earth and in space. Okoye (Danai Gurira) mentions that there was a massive earthquake under the ocean, but it's sort of offhand, presumably because there's not a lot that the Avengers can do to fight natural disasters. Originally, fans speculated that this might lead to the appearance of Namor and the Marvel version of Atlantis since that character is ocean-based.
Post-Eternals, Reddit user Emergency-Hat6374 suggested the earthquake might have had something to do with the Emergence. According to Ajak's explanation, the Emergence was delayed by the Blip, but it seems at least possible that the oceanic earthquake in Endgame was related to it. The timeline of the Emergence could have been dragged out, meaning that the preceding earthquakes happened for longer before the event.
Then again, Ajak also explains that everyone returning from the Blip is what caused the Emergence this time. The population is, once again, large enough to trigger the event, and Ajak knows for a fact that it has. As some fans pointed out, earthquakes in the ocean are very common and the one mentioned during Endgame could just as easily have been a natural occurrence that was unrelated to the Celestials.
To see how the Emergence affects Earth, Eternals is playing in theaters.