Atop the tallest mountains in the world Tallest if you measure from where they actually start, at the bottom of the ocean, of course! The Big Island has two big mountains, Mauna Kea (inactive volcano) and Mauna Loa (active volcano, last erupted in 1984). It's pretty easy to get to the top of Mauna Kea (there's a road) but for Mauna Loa, you have to walk.