There's already been much written about the film serving as an allegory for depression and the tireless war the mentally ill wages against themselves.
For starters, every member of the team has a depressive affliction which drives them towards the possibility of self-destruction, from Lena's infidelity to Ventress' cancer, Anya's (Gina Rodriguez) alcoholism, Josie (Tessa Thompson) being a cutter and Cass (Tuva Novotny) mourning her dead daughter.
The Shimmer itself feels like a literal manifestation of depression's oppressive power, be it the fact that it's a literal bubble that exerts influence on those inside it, or the fact that it alters conventional reality and causes time to pass in a haze (something any depressed person can account for).
This is realised most literally, and most affectingly, in the third act when Lena runs for the lighthouse door to escape, but finds herself being pressed up against it by the mimic. Though an initial reading of the scene would seem to suggest the mimic is trying to kill her, it's clear on reflection that the mimic is just echoing her own fleeing behaviour, resulting in Lena being damn-near suffocated by herself (again, self-destruction).
In order to prevent being literally killed under her own weight, Lena has to let go and stop fighting the tide. Conversely, the one suicidal member of the group, Josie, also decides to let go, but rather than give in to her deathly impulses, she's able to transmute into something else: a plant vestige of her former self. Her depressive tendencies literally sprout new life, it seems.
Sci-fi has been a ripe genre for highlighting existential angst in all its forms, from a fear of the unknown to questions about what it means to be human, and Garland's film concludes as a potent statement on the internal struggle so many humans face every day.
But with the film's examination of emotional turmoil in mind, let's look at how it confronts the more tangible, physical threats that humanity faces...
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