Radioactive material is a dangerous block that is left in the wake of nuclear explosions. Resembling a green form of mycelium, this block has the ability to slowly spread to other adjacent grass or mycelium blocks. It only spreads at a rate of a few blocks per minecraft day, however, and occasionally reverts to mycelium.
Any mob or player that comes within a radius of 3 blocks of this substance has a chance (guaranteed with direct contact) to contract Radiation III, a lethal status effect which lasts for a full minute (3 times as long as you will). This is curable with an Antidote