It all started with the discovery of a three meter high monolith in Utah on November 18 during a helicopter survey of bighorn sheep.
Dutch journalist Nouska du Saar, who specialises in open-source intelligence, used Maxar satellite images to determine that the monolith appeared between July 7, 2016 and October 21, 2016.
The Utah monolith disappeared on November 27, as mysteriously as it had appeared.
Since the discovery, similar monoliths have appeared in California, Colombia, Nevada, New Mexico, Pennsylvania, Texas, Canada, United Kingdom, Belgium, Spain, Germany, Netherlands and Romania (up to December 9).
The first (Utah) monolith was compared to works by artist John McCracken (1934–2011), who lived in the southwest desert, believed in the existence of extraterrestrials, and expressed an interest in leaving behind a piece of artwork in the desert. Other artists have also been identified as possibilities (see Wikipedia link below).
Meanwhile, a community of artists collectively known as The Most Famous Artist have claimed to be behind these structures, and are selling them for $45,000 a piece.
The alien connection, as also the term monolith given to these objects (and almost certainly their inspiration) owes itself to Stanley Kubrick's 1968 film 2001: A Space Odyssey based on Arthur C. Clarke's science fiction novel of the same name. He also wrote the screenplay for the film.