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 t here were no injuries.
Five fire crews attended the incident, which happened at around 11am today.
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."
He later added: " As part of routine in-service testing conducted on behalf of the MoD, a planned and remotely controlled test took place at MoD Shoeburyness today.
"A contained fire occurred at one of the environmental test centre buildings on site following the test. No-one was injured.
"The emergency services were called as part of standard procedures, their assistance was not required and they were subsequently stood down. An investigation is under way as part of our standard safety procedures."