Two 15-year-old boys have suffered 2nd degree burns in an explosion inside a World War II bunker in Oxfordshire.
It is thought the teenagers were smoking inside the pillbox off London Street, near Faringdon, on Sunday.
One of the boys threw a cigarette into a bucket sparking a "flash fire".
Police said they were in a stable condition at Bristol's burns unit. Officers are hoping to speak to them later.
An investigation into the incident has been started involving the fire service and military munitions experts.
Inspector John Turner of Thames Valley Police said: "There aren't unexploded incendiary devices in the area, it is safe, it has been confirmed that there was some powder, it could be that it was fireworks, we don't know yet."