Olympus Mons, the colossal shield volcano on Mars, is believed to have formed during the early Hesperian period of Mars, around 3 billion years ago. It was created by magma from Mars's mantle that began to swell upwards, breaching the planet's crust and building the massive volcanic mountain over eons of volcanic activity. Some studies suggest its bulk formation occurred between about 3.67 billion and 3.53 billion years ago. Its most recent large volcanic eruption is estimated to have happened around 25 million years ago. This volcano stands nearly three times the height of Mount Everest and is the tallest mountain currently discovered in the Solar System.