The Lake Washington sunken forests were both a part of the scientific discovery of a major fault line under Seattle, Washington, and part of a timber piracy case in the late 20th century. In a precedent-setting case, the Washington State Supreme Court decided that ancient drowned forests are state property and not eligible for salvage.
Creation of the sunken forests
A major earthquake (magnitude 7 to 7.5) occurred on the Seattle Fault about 900 C.E., creating multiple forms of evidence that led to the discovery of the fault and its description in Science in 1992. One of the forms of evidence was landslides at three locations around Lake Washington: off the southeast corner of Mercer Island (47°31′30″N 122°13′15″W), on the west side of the island across from Seattle's Seward Park (47°33′45″N 122°14′00″W), and one near Saint Edward State Park in present-day Kirkland, Washington (47°42′30″N 122°15′15″W).
The landslides on heavily wooded land created "bizarre submerged forests" of old-growth timber, preserved by the cool water and low oxygen in the deep lake. These sunken forests were known to early European settlers of the Seattle area, for whom the snags could be a hazard to ships on the lake, and as early as 1919, nearly 200 of the sunken trees had been removed from depths of 65–132 feet (20–40 m). Radiocarbon dating and tree-ring dating of the wood in the late 20th century helped to establish the date of the earthquake.