What are bunker busters, one of which Israel used to kill Hezbollah chief?

Israel used a US-made 900-kg Mark 84 series bomb in airstrikes that killed Hezbollah chief Hassan Nasrallah. Also called bunker busters, the munitions are specially designed to penetrate hardened targets and can penetrate up to 30 metres of Earth. They're integrated with delayed fuses to ensure they explode only after penetrating target. Their concept dates back to World War II.