Mount Pinatubo

The Philippine volcano whose 1991 eruption cooled the entire planet/Volcanoes

Mount Pinatubo's cataclysmic 1991 eruption was the second largest of the 20th century, ejecting roughly 10 cubic kilometers of material and sending a sulfur dioxide cloud around the globe. The aerosol veil lowered global temperatures by about 0.5 degrees C for two years. Before the eruption, nobody paid it much attention, which is exactly the kind of thing a volcano would do before ruining everyone's day.