Tamu Massif

The Tamu Massif is an extinct submarine shield volcano in the northwest Pacific Ocean. It looks like a hybrid between a mid-ocean ridge and a shield volcano. 
In 2013 researchers announced that it could be a single volcano. If true, that would make Tamu Massif the largest known volcano on Earth. 
The Tamu Massif was formed about 145 million years ago during the late Jurassic to early Cretaceous period over a relatively short period of time (a few million years) and then became extinct.
Tamu Massif is in the Shatsky Rise about east of Japan. The volcano covers an area of about 553,000 square kilometers or 214,000 square miles. Its summit is about below the surface of the ocean, and its base extends to about deep. The volcano is about high, standing on the floor of the ocean.