text: The blaze started at the ICL UK Boulby mine at 00:30 BST on 13 April, 1,100m (3,600 ft) under the sea bed and five miles (8km) from land.
  Seven workers were taken to hospital and treated for smoke inhalation.
  The Health and Safety Executive (HSE) said a notice had been served in relation to the underground fire.
  More than 100 staff were underground at the time, 11 of whom were temporarily unaccounted for.
  Polystyrene blocks are believed to have caught fire in the mine.
  A spokesman for ICL UK, said: "We have been working closely with HM Mines Inspectorate in the investigation into the recent incident.
  "Safety is always our first priority and we are fully committed to working with the inspectorate in order to identify ways in which procedures can be improved.
  "We will, of course comply with all the measures set out in the improvement notice."
  The company has until 27 June to comply with the notice.
  The HSE said the investigation was ongoing.
  One of Europe's deepest mines, the site employs about 1,100 people.
  Last year, the company announced it was shedding hundreds of jobs as part of a "new business strategy" to produce fertiliser polysulphate.
target: A potash mine in East Cleveland has been issued with an improvement notice after a fire broke out hundreds of metres below the sea bed.