Europe’s brutal heatwave has already sparked record-breaking temperatures, deadly wildfires and mass evacuations. Now a new crisis looms: an energy shortage.

Hungary’s Prime Minister Péter Magyar warned on Monday that the “hardest five days are ahead of us” as the country’s sole nuclear power station teeters on the verge of shutdown due to record-low water levels in parts of the Danube River.

“We are in uncharted territory,” Magyar said in a video address. “As water levels continue to recede, the possibility of shutting down the plant is still there.”

Later Monday, Magyar said “water management forecasts” for the coming days had become “slightly” more optimistic.

The expected closure of the Paks Nuclear Power Plant would be its first since the facility came online 44 years ago and risks a widespread energy crisis, he said. Water from the Danube is used to cool the facility, which produces nearly half of the country’s electricity.