Hydrogen ignition during the (illegal) filling of a balloon.Two men had decided to fill in a weather balloon with hydrogen in a storeroom located in the basement of a block of flats. Their idea was to attach a video camera to the balloon for the purpose of taking aerial photos. To fill in the balloon, the men had purchased a 20 kg hydrogen bottle and a valve and constructed a special mouthpiece for the purpose. They had tried filling a similar balloon previously. Then, they did not experience any problems in filling the balloon and had left the balloon in the basement to let it empty. Before filling the balloon, they had opened all the vents in the basement. When the balloon had inflated to a diameter of about one metre, it suddenly exploded. A woman and two children were also present in the room during the fill-up. All those present sustained burns in the explosion. The explosion damaged the block of flats in several ways: the main entrance door was torn off, nearly all of the doors and fire doors along the corridor cutting across the building basement were damaged, and one of the masonry partition walls fell down.