On 14 July 1876, "Thunderer" suffered a disastrous boiler explosion which killed 45 people. One of her boilers burst as she proceeded from Portsmouth Harbour to Stokes Bay to carry out a full-power trial. The explosion killed 15 people instantly, including her commanding officer; around 70 others were injured, of whom 30 later died. This was the Royal Navy's most deadly boiler explosion through the whole century. A model representing the failed boiler was made and is now in the Science Museum, London. The explosion was caused because a pressure gauge was broken and the safety valve had corroded in place. When the steam stop valve to the engines was closed, pressure in the boiler rose and could not be released. The four box boilers were the last in service in the Navy and operated at what would even then would have been considered a relatively low pressure, for more modern boilers, of . The boiler was repaired and the ship was completed on 26 May 1877 at a cost of £368,428.