This is a map of the ocean floor.
Click on image for full size
Image from: NOAA/NESDIS/National Geophysical Data Center, Boulder, CO
One of the most powerful volcanic explosions in the history of the world occured at Krakatoa in the last century. Krakatoa was formerly a volcanic island between Java and Sumatra.
In August 27, 1883, a gigantic explosion blew the island apart.
The large explosion was due to super-hot steam, which was created when the walls of the volcano broke and let sea water into the magma chamber.
The island exploded with the force of a 100 megaton bomb. The explosion was heard as far away as Madagascar (2,200 miles). Ash from the explosion rose so high that it affected weather patterns for the next year. Tidal waves from the explosion destroyed 163 villages along the coast of Java and Sumatra.
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