Krakatoa's eruption was the loudest noise in recorded history, heard as far away as Perth, Australia. That's about 2,000 miles -- basically, imagine being in New York City, and hearing something that happened in Utah. You'd probably see it on the news long before you actually heard it since it takes sound nearly three hours to travel that distance.
The blast was the equivalent of 200 megatons of TNT. For perspective, the largest explosion ever made by humans was the detonation of a Russian hydrogen bomb, which was 50 megatons. That blast broke windows in buildings 560 miles away. Krakatoa was four times that; the cloud it generated wiped entire villages off the map 25 miles away and created a tsunami that traveled all the way to South Africa. That wasn't all Krakatoa's neighbors got for their birthday that year; giant pieces of rock and coral reef fell from the sky as well.
Via Wikimedia Commons
No really, "giant piece of coral reef" was literal.
Krakatoa ejected so much debris into the upper atmosphere that it changed the weather for five years. Much of what came out of the explosion was sulfur, which reached the stratosphere and reflected out more sunlight than regular water vapor, dropping the global temperature by just over 2 degrees. And since blowing up and drowning vast swaths of the planet wasn't a big enough screw you, it fell back to earth as rain -- acid rain.