In 1815, Mount Tambora erupted and its emissions spread across the globe, blotting out the sun for almost an entire year. This wreaked havoc on agriculture, leading to famines all across the Northern hemisphere. It was the year without summer— one of the darkest periods in human history. So why are some modern researchers considering repeating it? David Biello digs into geoengineering. [Directed by Léon Moh-Cah, Andi Concha, Na Na Na Studio, narrated by Addison Anderson, music by Stephen LaRosa].