The seafloor isn’t just a flat, sandy expanse. Just like Earth’s surface, the bottom of the ocean contains landforms, like mountains and valleys. By the 1950s, scientists had evidence that at least one mountain range—the Mid-Atlantic Ridge—ran along the ocean floor. But it wasn’t until Marie Tharp created detailed maps of this feature that a bigger picture began to emerge. She showed that the seafloor was much more complex than originally thought—and discovered a hidden ocean feature powerful enough to shift entire continents.