The incident happened in the early hours of Wednesday morning during planned maintenance, but no one was injured.
The mines inspectorate is investigating after a tunnel was flooded at Boulby potash mine. The incident happened in the tailings tunnel during planned maintenance.
In a statement ICL UK said: "In the early hours of Wednesday morning, a water ingress has occurred in the tailings tunnel, during a planned maintenance activity to replace two effluent valves.
"The maintenance team working on the valves, exited the tunnel and subsequently rode out as the water flow continued.
"The tailings tunnel has now flooded, and we are monitoring the level in the shaft. A dive team was already on station and will dive to inspect the outfall, once the water levels stabilise.
"Plans are currently being drawn up to recover the situation."
They added: "All mining operations will continue as normal, as will the salt and polyhalite plants, although the potash plant will not operate until an agreed method of effluent disposal is
established. The mines inspector has been informed and we are working closely with them."
The incident happened just weeks after the mine announced plans to apply to keep the site for 40 more years. The current planning permission has been in place since 1998 and expires in 2023.
The mine has had a troubled year - a safety probe was launched in June after a 56-year-old man named John Anderson died at the mine in an incident described as a sudden and powerful release of gas.