The reactor exploded early on April 26, 1986, sending a column of fire and radioactive debris high above the Chernobyl nuclear power plant in the Soviet Union.

The fallout settled over the streets, schools and playgrounds of nearby cities like Pripyat. Trace amounts of particles were eventually detected across the Northern Hemisphere. It was the world’s largest ever accidental release of radioactive material.

In the beginning, the Soviet Union said little about the explosion, which was triggered by a safety test and exacerbated by design flaws. Officials obfuscated and denied. After radiation levels rose in Sweden, Soviet officials acknowledged that something had gone wrong. But they hid the scale of the disaster for months, even from the thousands of people sent to fix it.

Firefighters, miners, soldiers and civilians — who would come to be known as “liquidators” — shoveled debris and sealed the shattered reactor, without fully understanding the risks. They sometimes paid with their lives.

Pripyat was evacuated 36 hours after the disaster. Thousands of other residents nearby were ordered to do the same in the weeks and months that followed. The city and dozens of villages were left to rot. This corner of the Soviet Union became known as the Chernobyl exclusion zone, an area roughly the size of Rhode Island that remains restricted. The Soviet Union fell apart; Chernobyl remains in Ukraine.

These photographs — some taken in the first days after the explosion, others many years later — illustrate the immediate response, the evacuation and what was left behind.