The Soviets have never been completely transparent about Chernobyl, but this is the story as best as I was able to synthesize it: It seems that the alarm system was malfunctioning (going off all the time) so it was turned off by the tech monitoring the system. He had the fuel rods pulled out of their cooling chambers for maintenance work, was distracted (remember, the alarms are off), then by the time he focused back on the task at hand the rods had begun to melt and couldn't be reinserted into their cooling chambers. The rest, as they say, is the stuff of infamy -- it was an act of pure human error. So baring idiocy or acts of terror, I am quite comfortable that nuclear reactors are quite safe. And the energy they produce and which modern society demands is free of greenhouse emissions. As you know some countries, like France, produce the bulk of their electricity using nuclear reactor technology.
What I've found amazing about Chernobyl is how quickly nature recovered around the site. Wolves returned, wild animals have (mostly) flourished, and so on. It has essentially begun to transform itself into a national park. There is a fine layer of radioactivity buried into the dirt, so essentially this can never be farmed or built on in the future.
Back to the question at hand. I'd add many of the engineering wonders of the 20th century to the list. The Brooklyn and Golden Gate Bridges, the Empire State Building, and so on. These were revolutionary leaps forward in technology and engineering.