You're now approaching Fishing Bridge, named for the bridge constructed here in 1902. The original rough-hewn corduroy log structure was replaced in 1937 with the current bridge, which is open to visitors. Fishing Bridge was declared a National Historic Landmark in 1987. Today, it is home to a store, food counter, gas station and an RV park -- for hard-sided trailers only. The Fishing Bridge Museum and Visitor Center provides information and exhibits on nearby birds, animals and geology. It also offers easy access to a sandy beach on beautiful Yellowstone Lake. This was once a major spawning site for cutthroat trout, so it was historically a popular place to fish. But overfishing and the introduction of a nonnative species, lake trout, threatened the native cutthroat population so much that the area had to be closed to fishing in 1973. Even so, it's a great spot to observe fish and waterfowl. The lake still has North America's largest population of wild cutthroat trout. How this species got here in the first place was a mystery for some time. We know that it came from the Pacific Ocean originally, but Yellowstone Lake drains into the Atlantic Ocean through its only outlet, the Yellowstone River, which flows from the northern end of the lake at Fishing Bridge. Scientists now think that, a long time ago, the lake drained south over the Continental Divide -- at Two Ocean Pass -- and then into the Snake River which, eventually, feeds into the Pacific. So the trout could have traveled up the Snake to find their way here. At 7,733 feet above sea level, this lake is North America's largest high-elevation lake. Its surface covers 132 square miles! This is part of the giant caldera created by huge volcanoes that erupted hundreds of thousands of years ago. After the explosions, the underground magma chamber collapsed, forming the caldera. Lava flows filled in some of it, but much of its shape still remains. Over the centuries, glaciers and other climatic forces added the final touches on the lake, filling it with freshwater. Scientists recently discovered that Mallard Lake Dome and Sour Creek Dome are once again rising -- at a rate of about one inch a year. Sour Creek Dome's growth is actually making Yellowstone Lake tilt toward the south. As a result, the sandy beaches on the north shore of the lake are getting bigger and land along the lake's southern edges is flooding.