The diameter of the largest known underwater volcano, Tamu Massif, is about 650 kilometers. More precisely, its broad, gently domed main structure measures roughly 450 by 650 kilometers, so 650 kilometers is the standard quiz answer for its maximum diameter.
Tamu Massif lies beneath the northwest Pacific Ocean on the Shatsky Rise, about 1,600 kilometers east of Japan. Its summit is approximately 1,980 meters below sea level, while its base reaches about 6.4 kilometers beneath the ocean surface. The volcano rises about 4,460 meters from base to summit.
Scientists believe Tamu Massif formed around 145 million years ago from extensive basaltic lava flows. Its name comes from the initials of Texas A&M University, where geologist William Sager conducted important research on it. Although its classification as one single volcano has been debated, it remains one of Earth's largest volcanic edifices by area.
A common mix-up is confusing its diameter with its height: 650 kilometers describes its width, not how tall it is.