The skies over an Indonesia province have become so polluted by widespread forest fires that they began to glow a post-apocalyptic red over the weekend.

Some 900,000 Indonesians have suffered respiratory problems caused by extensive forest fires this summer, which are believed to have been started by illegal burning to clear land for farming.

A toxic haze has blanketed parts of Borneo and Sumatra in the past few months, according to the local authorities, and has triggered diplomatic tensions as it spreads to regional neighbours, including Singapore, Malaysia and the Philippines.

In the Indonesian province of Jambi, on Sumatra’s east coast, the haze took on an eerie blood red hue on Sunday, as the sun’s rays were filtered by microparticles in the air, prompting unsettled residents taking to social media to express their concern.

According to the Straits Times, citing the Indonesia National Board for Disaster Management, the phenomenon is known as “Rayleigh Scattering” and is caused by the movement of haze away from hot spots.