How are illegal nuclear tests spotted?

It's not easy to hide a nuclear blast. Hundreds of monitoring stations exist in 89 countries around the world, sensing signs of nuclear activity and reporting back to the headquarters of the Comprehensive Nuclear Test Ban Treaty Organisation (CTBTO) in Vienna. The system also picks up non-nuclear activity, such as earthquakes and tsunamis. Indonesia has just signed up to the CTBTO system to ensure that the damage wrought by the devastating 2004 Indian Ocean tsunami is never repeated.