I came across this captivating article about a teenager who attempted to build a nuclear breeder reactor in the vicinity of Detroit. The most fascinating part of the story is how far he got, with very few resources other than ingenuity, determination, and a complete disregard for safety.
As technology progresses, this is our major extinction risk.
In the 1990s, this kid was able to obtain dangerous quantities of radioactive materials by laboriously extracting small amounts from readily available equipment.
In several more decades, some similarly ingenious, short-sighted kid is going to synthesize a virus that ends up killing most humans. Not because he intends that result, but in a blind attempt to do good, or to compensate for social maladjustment by showing off his prowess.
Our Earth is a single system where everything is interrelated. A monkey could destroy the planet if she knew what actions to take, and what order to perform them in. Increasingly, our knowledge is bringing us closer to the point where a single person, or group, will be able to connect the dots and unleash the destruction. People with both the brilliance to give them the ability, and the maladjustment to give them the will, may be few and far between; but it's only going to take that one person in 10 billion.
I wrote about this back in 2006, and years before that. We need self-sustaining colonies in space so that a mishap in one ecosystem doesn't end up killing us all.