Lake Crescent is a deep lake located entirely within Olympic National Park in Clallam County, Washington, United States, approximately 17 miles (27 km) west of Port Angeles, Washington on U.S. Route 101 and nearby to the small community of Piedmont. At an official maximum depth of 624 feet (190 m), also the maximum depth of the depth sounder used to find that depth (see depth section), it is officially the second deepest lake in Washington. Unofficial depth measurements of more than 1,000 feet (300 m) have been rumored in the region for years, although this figure has recently been proven false after a lake-wide bathymetric survey was performed from 2013 to 2014 by Eian Ray and Jeff Enge. The results of this survey showed the maximum depth as being 596 feet. Using GIS statistical analysis, this survey also showed the lake contains approximately 0.5 cubic miles of fresh water.
Lake Crescent is known for its brilliant blue waters and exceptional clarity, caused by a lack of nitrogen in the water which inhibits the growth of algae. It is located in a popular recreational area which is home to a number of trails, including the Spruce Railroad Trail, Pyramid Mountain trail, and the Barnes Creek trail to Marymere Falls. The Spruce Railroad Trail follows the grade of what was once the tracks of a logging railroad along the shores of the lake. Following this trail on the north side of the lake, one can find the entrance to an old railroad tunnel as well as "Devils Punch Bowl", a popular swimming and diving area.
The lake was formed when glaciers carved out deep valleys during the last Ice Age. Initially, the Lake Crescent valley drained into the Indian Creek valley and then into Elwha River. Anadromous fish such as steelhead and coastal cutthroat trout migrated into the valley from lower waters.
Approximately 8,000 years ago, a great landslide from one of the Olympic Mountains dammed Indian Creek and the deep valley filled with water. Many geologists believe that Lake Crescent and nearby Lake Sutherland formed at the same time, but became separated by the landslide. This theory is supported by Klallum tribe legend which tells a story of Mount Storm King being angered by warring tribes and throwing a boulder to cut Lake Sutherland in two, resulting in Lake Crescent.  The results of the landslide are easily visible from the summit of Pyramid Mountain. Eventually, the water found an alternative route out of the valley, spilling into the Lyre River, over the Lyre River Falls, and out to the Strait of Juan de Fuca.
The anadromous fish populations in the lake became landlocked, because those fish could not ascend Lyre River Falls, making a barrier in that direction. Over time, two different subspecies of fish evolved in the lake: the endemic Beardslee trout, a relative of rainbow trout, spawns in the Lyre River above the falls, while the Crescenti Cutthroat Trout spawns in Barnes Creek.
In the early 1960s, the U.S. Navy did a survey of the lake using a Furuno depth sounder. They were not able to verify the maximum depth on their equipment. During a 1970 depth survey conducted by the students of the fisheries program at Peninsula College in Port Angeles, Washington, students used instruments that could not record measurements beyond a depth of 624 feet, which thus became the "official" depth of the lake as recorded by the National Park Service. However, when power cable was being laid in the lake in the 1980s, instruments showed depths in excess of 1000 feet, the maximum range of the equipment used.
The Lake Crescent Bathymetric Survey: In 2013 and 2014 geographic data scientists Eian Ray and Jeff Enge performed a lake-wide bathymetric survey, taking over 5,000 depth soundings. GIS statistical analysis showed the lake contains approximately 0.5 cubic miles of fresh water. The deepest spot was shown to be 596 feet deep. Much of the shoreline of the lake drops off steeply, in many cases a shear underwater cliff face. During the Lake Crescent Bathymetric Survey it was speculated that the erroneous depth record of 1000 feet from the 1980s was a result of the sonar signal reflecting off the steep underwater slopes near the shore.
It is not certain whether the lake was named for its crescent shape or for its proximity to Crescent Bay, which was named by Henry Kellett in 1846. In 1849 two British–Canadian fur trappers, John Sutherland and John Everett, forged inland from Crescent Bay. The two lakes they found became known as Lake Sutherland and Everett Lake. Later, Everett Lake was renamed Lake Crescent. It has also been known as Big Lake and Elk Lake.
In 1890, while the Port Crescent Improvement Company was promoting its townsite near the lake, M.J. Carrigan started the Port Crescent Leader to help boost the town. He wrote of the beautiful lake, which he called Lake Crescent, and the name soon became well established. 
In 1929, Russell and Blanch Warren disappeared while driving in the vicinity of Lake Crescent. Their whereabouts remained unknown until 2002, when their 1927 Chevrolet automobile was found over 160 feet beneath the surface of Lake Crescent.  Also, in 1937, a waitress named Hallie Illingworth went missing, and was found three years later by local fishermen. She was apparently weighted down, and over time her restraints had decayed, allowing her corpse to float to the surface. The corpse was completely preserved by the near-freezing lake temperature. Her skin had turned into a substance described as "ivory soap," which was caused by minerals in the lake water interacting with Illingworth's body fat in a process called saponification. Her husband, Montgomery J. "Monty" Illingworth, was later convicted of the murder. He served nine years in prison, until he was paroled in 1951.