Olympus Mons is the Mauna Loa of Mars. Like the mountain on the island of Hawaii, it’s the largest volcano on its planet —in this case, Mars. Also like Mauna Loa, it built up as molten rock bubbled through a “hotspot” in the crust. Finally, Olympus Mons might once have stood in the middle of an ocean.Olympus Mons is almost 13 miles high, and covers an area the size of France – much bigger than Mauna Loa. That’s because it never moved away from the hotspot, so it kept on building for billions of years. Today, it’s either dormant or extinct.It stands on a wide base that’s about four miles high, with sheer cliffs all around. A recent study says the top of the base shows evidence of contact with water.Scientists have already seen the possible shoreline of a shallow ocean. The ocean vanished long ago, as Mars grew colder and lost most of its air.The study found features along the rim of the Olympus Mons base that look like they formed when lava spilled into liquid water – supporting the idea of a long-gone ocean. So Olympus Mons might once have been not only the biggest volcano on Mars, but the biggest island as well.Mars is inching into the dawn sky. It’s quite close to the Sun, though, and it rises at a shallow angle. If you’re in Hawaii or southern Florida or Texas, you might spot it quite low in the southeast before sunrise. The rest of the U.S. won’t see it for a few weeks.More about Mars tomorrow. Script by Damond Benningfield