A woman in India captured footage of a curious rectangular shape resembling a glowing doorway that appeared in the sky over a city and no one is quite sure what could have created the eerie anomaly. The strange scene was reportedly filmed this past Sunday evening as Aditi Singh was returning to her residence in the city of Bengaluru. Sharing her video of the odd event on Instagram, she explained that “my friend was dropping me back home” when they noticed something rather unusual above them in the form of a distinctly rectangular shadow-like shape that seemed to glow around its edges.

Marveling that the very weird aerial anomaly did not appear to be “a natural phenomenon,” Singh observed that “the clouds were running against it like there is something levitating in the air.” The bewildered witness also noted that “the moon was in the waxing crescent phase,” which made the considerable degree of illumination surrounding the oddity to be particularly inexplicable. According to Singh, the strange sighting lasted for approximately 90 seconds before the anomaly vanished and “the sky was just filled with clouds.” Left with no explanation for what had appeared in the sky, she ultimately mused that “I have so many questions” and asked if anyone could identify a cause for the rectangular shape.

Singh’s video quickly went viral after it was shared on social media and viewers had all manner of suggestions for the nature of the peculiar anomaly. Some imaginative individuals speculated that it was some kind of doorway or portal that briefly manifested before disappearing. Others theorized that perhaps it was the shadow of a building or a light projection from some event being held in the city. Another plausible possibility put forth by people online is that the anomaly is an optical illusion known as a Brocken spectre.

A Brocken spectre (British English; American spelling: Brocken specter; German: Brockengespenst), also called Brocken bow, mountain spectre, or spectre of the Brocken is the magnified (and apparently enormous) shadow of an observer cast in mid air upon any type of cloud opposite a strong light source. Additionally, if the cloud consists of water droplets backscattered, a bright area called Heiligenschein, and halo-like rings of rainbow coloured light called a glory can be seen around the head or apperature silhouette of the spectre. Typically the spectre appears in sunlight opposite the sun’s direction at the antisolar point.

The phenomenon can appear on any misty mountainside, cloud bank, or be seen from an aircraft, but the frequent fogs and low-altitude accessibility of the Brocken, a peak in the Harz Mountains in Germany, have created a local legend from which the phenomenon draws its name. The Brocken spectre was observed and described by Johann Silberschlag in 1780, and has often been recorded in literature about the region.