Ukraine marked the 30th anniversary of the nuclear disaster in Chernobyl on Monday, sounding sirens at the same time as the first explosion at the reactor on April 26, 1986. The disaster is considered one of the worst nuclear accidents in history. A botched nuclear test at the plant in then-Soviet Ukraine sparked a meltdown that caused clouds of radioactive material to drift across Ukraine, Russia, Belarus, and parts of northern Europe. The accident left 31 workers and firemen dead, but thousands more have suffered radiation-related illnesses over the past three decades. Some former residents returned to the area while a memorial service is being held in Slavutych, a town that was built to resettle workers who lived near the plant. A church service is also being held in Kiev for families of the victims.