The 20th century saw by far the greatest extent of tunnelling when the Rock was turned into a huge underground fortress capable of accommodating 16,000 men along with all the supplies, ammunition and equipment needed to withstand a prolonged siege. The tunnelling finally ceased in 1968 when the British Army's last specialist tunnelling unit was disbanded. Since then, the tunnels have progressively been turned over to the civilian Government of Gibraltar, although a number are still owned by the Ministry of Defence and some have been sealed off entirely as they are now too dangerous to enter.