Most people know of Mount Vesuvius and its great explosion that wiped out most of Pompeii, but who knows of the Indonesian eruption that plunged the globe into 18 month darkness?
Here's what the Year without a Summer was, and how it occurred.

The Explosion

In April 1815, Mount Tambora, located in modern-day Indonesia, erupted. The eruption was the most powerful volcanic eruption recorded in human history, reaching a 7 on the Volcanic Explosivity Index. It ejected around 41 cubic kilometres of DRE (dense-rock equivalent) material into the atmosphere.

Even after its initial explosion, it continued unleashing small Phreatic eruptions for the next six months to three years. The ash from the eruption column spread across the globe, causing the infamous 'Year without summer' in 1816.

In these 18 months of shadow, average global temperatures dropped by approximately 1 degree celsius. The eruption took place at the end of a global cooling period known as the 'Little Ice Age', and the eruption only intensified the cooling effect of the already cool Earth.

Effects on the Earth

China faced lots of crop failure, along with flooding of crops. Summer snowfall was reported in areas with tropical climates, and there was a large-scale famine which brought about the decline of the Qing Dynasty.

Japan had already been recovering from a previous famine, but they did not record or report any crop failings or famines caused as a direct result of the eruption.

India faced torrential rains that boosted the spread of the disease named Cholera.

In Europe, countries were still recovering from the Napoleonic Wars, which meant the effects of the eruption were much heavier. Torrential rains and low temperatures were common in the UK during this period, and a widespread famine occurred across the continent. It was the worst famine of 19th century mainland Europe.

Major Typhus epidemics spread across Europe, and Huge storms and abnormal rainfall was recorded in this time period. Hungary even experienced brown snow due to the ash in the atmosphere.

A persistent 'Dry Fog' was observed in the eastern part of the United States which dimmed the sunlight so much that they were able to see sunspots. Corn and Grain prices rose dramatically due to the shortage.

"It is now the middle of July, and we have not yet had what could properly be called summer. Easterly winds have prevailed for nearly three months past ... the sun during that time has generally been obscured and the sky overcast with clouds; the air has been damp and uncomfortable, and frequently so chilling as to render the fireside a desirable retreat." -Columbian Register (New Haven, CT), July 27, 1816

After the Eruption

Even after the eruption, a haze continued to hang in the sky after the eruption due to the high levels of Tephra in the atmosphere. The sunsets were also a much richer red in a way they had not been before the eruption.

Some people believe that the Year Without A Summer was a preview of what would happen in the years to come.
Some believe it didn't happen at all.