The Mauna Loa is the world's biggest active volcano but only is less than two per cent of the size of the Tamu Massif.

By Danielle Demetriou, Tokyo 2:33PM BST 06 Sep 2013

The world's largest volcano spanning the size of the
entire British Isles has been discovered on the floor of
the Pacific Ocean.
The volcano, located 1,000 miles east of Japan and
dubbed the Tamu Massif, is the largest on earth as
well as one of the biggest within the solar system.
The sprawling structure covers 119,000 square miles,
rising around 2.2 miles above the bottom of the sea floor and delving 18 miles into the crust of the earth.
The volcano, which is not believed to be active,
consists of a vast single rounded dome in the shape
of a shield which was believed to have been formed
144 million years ago when it last erupted.
The epic dimensions of the Tamu Massif almost
match the Olympus Mons volcano on Mars, widely
regarded the largest volcano ever recorded,
according to scientists.
It also eclipses Hawaii's Mauna Loa, the world's
largest active volcano on Earth, which covers only
2,000 square miles, less than two per cent of the size
of the Tamu Massif.
Until now, it was originally thought that the underwater
formation, first discovered 20 years ago on an
underwater plateau called the Shatsky Rise, consisted
of a network of multiple volcanoes.
However, a team of geologists assembled data to
dispel this assumption and claim to prove that it is in
fact not only a single volcano, but among the largest
within the solar system.
"We show that the Tamu Massif is a single, immense
volcano, constructed from massive lava flows that
emanated from the volcano centre to form a broad,
shieldlike shape," the scientists conclude in Nature
Geoscience journal, which published the findings.
"We suggest that the Tamu Massif could be the
largest single volcano on earth and that it is
comparable in size to the largest volcano in the solar
system, Olympus Mons on Mars." The study was
conducted by a team of geologists led by William
Sager of the University of Houston – the scientist who
first discovered the Tamu Massif formation 20 years
ago.
The geologists made their discovery after drilling into
the ocean floor in order to obtain data from rock
samples and creating a chart of the seabed using
deep-penetration seismic scanners from a survey
ship.
The possibility that there were other undiscovered
super volcanoes lurking beneath the earth's oceans
could not be dismissed, according to Dr Sager, who
previously worked at Texas A&M University, after
which it was originally named.
"We don't have the data to see inside them and know
their structure, but it would not surprise me to find out
that there are more like Tamu out there," he said.