The two men, aged 38 and 26, suffered serious facial burns in the explosion at the former Welbeck Colliery site, near Meden Vale, Nottinghamshire, on Saturday.
Their names had not been released last night, but emergency services said they were being treated in the Queen’s Medical Centre, Nottingham, where both were said to be in a critical but stable condition.
A spokesman for Nottinghamshire Police said: “It is understood that the men were working at the site as security personnel.
“Detectives are liaising with the site owners, UK Coal, with the fire service and the Health and Safety Executive to determine the sequence of events leading up to the incident.”
The explosion was reported at around 7:30pm and involved an electrical sub-station above the ground.
A spokeswoman for Nottinghamshire Fire and Rescue Service said the explosion was in an electrical component in the single-storey building.
She added: “The fire itself was very small. The main issue was the explosion itself and the two people injured. Investigations are ongoing.”
According to UK Coal, Welbeck has not been a working colliery since May 2010.
Shafts have been filled and a small team has remained on site decommissioning the mine, which has exhausted three seams of coal since work on sinking the twin shafts began in 1912.
In its heyday, Welbeck employed around 1,400 miners and had an annual output of around 1.5 million tonnes.
A spokesman for UK Coal said it was thought the two security guards were the only people at the site at the time of the explosion and it was they who had called the emergency services after the explosion.