This is one of those films that is important to the development of film language, like Breathless or Pulp Fiction. It's about being empty, missing something, and trying to fake it by replacing what's not there with things you wish were, or think should be.
Where L'Eclisse feels more like a poem of disillusionment, L'Avventura feels more like a melodrama or, at the very least, like a slightly awkward transition between his neorealist melodramas and his more understated works. I do wonder if people would continue to regard this as his masterpiece if the scene they filmed where the drowned body is discovered would have been included in the final cut.
They simply do not make films like this anymore. A fascinating deconstruction of desire, lust, love, purpose, and existence. Similarly to Blow Up, Antonioni is making major statements about lifestyle and search for meaning. His is a master at composition and a truly methodical filmmaker. He says so much by saying so little. The film is not about the answers or plot, it's about it's characters and their questions.
When it gradually dawns on you that Anna's gone, there remains this subtle tension of expectation for her return throughout the film. Not able to grasp the loss, Sandro and Claudia try to resume their normal lives, but it's clear they can't. Apart from some spontaneous small moments of joy—the lovely church bells scene comes to mind—they are mourning, as becomes clear to hem in the beautiful final scene. Masterpiece.