What is the largest volcano on Earth by volume?

Mauna Loa on Hawaii, at something like 75,000 cubic kilometres of rock - though Tamu Massif, an underwater feature in the Pacific, was proposed as larger in 2013 and then reassessed in 2019 as a mid-ocean ridge structure rather than a single volcano, which is a fair illustration of how unfinished the sea floor still is.

Mauna Loa rises about 4,170 metres above sea level and roughly 9,000 metres above the sea floor - and then keeps going, because its weight has pressed the ocean crust down by several kilometres beneath it. Measured from the bottom of that depression it is over 17,000 metres of mountain, considerably more than Everest by any reckoning.

Hawaii exists because the Pacific plate is sliding over a stationary hot spot in the mantle. Each island is a volcano built while it sat over the plume and then carried away to erode and subside, so the chain is a conveyor belt with the ages increasing to the northwest - and the next island, Kamaehuakanaloa, is already building underwater to the southeast. It last erupted in 2022 after 38 years of quiet.