Four weapons testers were rescued from a bunker by an armoured vehicle after an explosion at a Ministry of Defence site.
According to Essex Fire Service, around 22lb (10kg) of explosives were involved in the blast, which occurred at a weapons testing centre at MoD Shoeburyness, near Southend.
The weapons testers were freed by MoD officers in an armoured personnel carrier, but there were no injuries.
Five fire crews attended the incident, which happened at around 11am today.
A fire service spokesman said: "Crews will remain standing by at this incident until specialist scientific teams, who have been called in to deal with the technical aspects of this incident, give them the all clear.
"Four weapons testers... have been brought to safety by MoD officers in an armoured personnel carrier."
A spokesman for QinetiQ, the private defence contractor that runs the Blackgate Road munitions centre, said: "No-one was hurt in the incident at our environmental test centre and we are conducting a full investigation into what happened as is part of our standard safety procedures."
Although Shoeburyness is a military testing site there are civilians who live within its perimeter, the Telegraph reports.