Technology
At the dawn of the 20th century, chemists dreamed of extracting nitrogen from the air and turning it into a limitless supply of fertiliser. Sceptics thought they were crazy -- it was possible in theory, but it was unclear if it could be done in practice. What happened next changed the course of 20th-century history, and provides inspiration to innovators pursuing a different dream today: sucking carbon dioxide out of the air to avert climate change. Might they not be quite so crazy after all?