A miles-long microalgae bloom in the Mediterranean Sea has been blamed for temporarily knocking offline many of Israel’s major desalination plants this week, threatening its main source of drinking water, CNN reported.

Five of the country’s six coastal desalination plants — which convert seawater into drinkable water — shut down on Sunday, according to Mekorot, Israel’s national water company. Since then, some of these facilities, which are privately owned and operated, have partially resumed operations, but only one was operating at full capacity on Tuesday.

“An event of this kind has never occurred before in Israel … it was an extremely rare event,” said Lior Gutman, a spokesperson for Mekorot, although he stressed in roughly 99% of cases, there has been no drinking water shortage.

Still, it’s an alarming situation given the country’s heavy dependence on desalination for drinking water. Israel is arid and has neither big rivers nor abundant rainfall, but its mastery of this technology has made it a water superpower. Desalination now provides at least 65% of its drinking water, with most plants located on the Mediterranean coast.

The problems started at the weekend, as a “sporadic, vast event of contaminates occurred across the Israeli coastline,” said Edo Bar-Zeev, an associate professor in water research at Ben-Gurion University, in Israel. When he and other scientists looked closer, they discovered it was microalgae, tiny organisms that feed on sunlight, carbon dioxide and nutrients in the water — and which can grow rapidly.

The microalgae bloom — stretching more than a dozen miles at least, according to Bar-Zeev — is believed to have originated in the Nile Delta and traveled northward with the sea’s currents up Israel’s coast. This is “the first time I saw something in this extent,” said Bar-Zeev, who has been testing the coastal water for 20 years.