It's a 10-mile blue crescent of fresh water that used to be a saltwater fiord.
Our forebears used it to float ancient tree trunks to the sawmill.
We admire it, splash in it, boat on it and fish in it. We also drink from it while we pollute it and argue about how to protect it.
It's a precious resource and a natural wonder.
It's Lake Whatcom, one of the defining features of Bellingham and the county that shares the lake's name.
Here are some things you may want to know about it. Most of the information comes from the city of Bellingham's website, which presents a vast amount of lake information at cob.org/services/environment/lake-whatcom.
The history information comes from a 2008 report to city by Becky Peterson of Geneva Consulting.
Lake Whatcom holds about 250 billion gallons of water, or the equivalent of about 379,000 Olympic swimming pools.
The lake is about 10 miles long and over a mile wide at its widest point, with a total shoreline length of approximately 30 miles.
The lake covers some 5,000 acres, with 8 percent within city limits. The lake's watershed covers about 36,000 acres, of which 3 percent is within city limits.
The average elevation is approximately 314 feet above sea level. The maximum lake level, set by a 1953 court ruling, is 314.94 feet. The city of Bellingham controls the lake level using a control dam at the head of Whatcom Creek.
Lake Whatcom is fed by 36 tributaries, among them Silver Beach Creek, Carpenter Creek, Olsen Creek, Smith Creek, Anderson Creek and Austin Creek. Other creeks flow intermittently.
The lake also receives water diverted from the middle fork of the Nooksack River via a city pipeline.
Lake Whatcom drains into Bellingham Bay via Whatcom Creek.
Lake Whatcom is the drinking water supply for about 100,000 county residents - about half of the county's population.
A few hundred homes draw water directly from the lake, but most people get their lake water from the city of Bellingham or from the Lake Whatcom Water and Sewer District system that serves 3,744 customers outside the city limits in Geneva, Sudden Valley and the lake's north shore.
The city of Bellingham withdraws water from the lake's midsection and sends it via a 1,200-foot pipeline to a treatment plant in Whatcom Falls Park.
The population of the Lake Whatcom watershed - the area that drains into the lake - is roughly 15,000 people, in about 6,500 homes.
Smallmouth bass, introduced by the state's game department in the early 1980s, provide the best fishing opportunities. (But see the Troubles section below before you toss one in the frying pan.)
The lake also contains native kokanee, cutthroat trout and longnose sucker, plus brown bullheads and largemouth bass that appear to have been introduced illegally.
The lake is a hot spot for bird-watching, with 125 species identified.
Forty-nine mammal species have also been spotted, including nine species of bat. All but the sharpest of observers are likely to miss the vagrant shrew (Sorex vagrans), southern red-backed vole (Clethrionomys gapperi), and the Pacific jumping mouse (Zapus trinotatus).
Ten amphibian species and two reptile species (both garter snakes) also have been recorded.
Lake Whatcom was formed by glaciers at the end of the last ice age, when ice was more than a mile deep in western Whatcom County.
Just after the ice melted, geologists believe, the lake was more like a saltwater fiord connected to the sea. As land compressed by the weight of the ice slowly rebounded, the fiord was cut off from the sea and became the freshwater lake we know today.
Kokanee in the lake are refugees from those saltwater days: Kokanee are landlocked sockeye salmon prevented from migrating to and from saltwater by the waterfalls in Whatcom Creek, downstream from the lake.
A Coast Salish village called Hahch-wah-ahm-eck once stood at the south end of the lake. The people who lived there were called Kaw-tcha-ha-muk.
White settlers began to move in during the 1850s, and the first private ownership claim was filed in 1858. By 1899, all the land around the shore of the lake had been claimed.
Small towns flourished, or tried to, at the end of the 19th century. Perhaps the largest was Blue Canyon City, with a population reported at 1,000 at its peak, thanks to the Blue Canyon coal mine that opened in 1891.
In 1895, an explosion at the Blue Canyon mine killed 23 men. A memorial to them still stands at Bayview Cemetery.
In 1901, the Bellingham Bay and Eastern Railroad connected the mine to Bellingham. Blue Canyon faded into oblivion after the mine closed in 1919.
The steamboat Rose went into service on the lake in 1890, carrying passengers from Silver Beach to Blue Canyon and other places.
By 1892, an electric railway rattled along from Holly Street in today's downtown Bellingham to the Silver Beach area at the north end of the lake.
In the early 1900s, the community of Park at the southeastern tip of the lake was big enough to have a school. Park was located on the Bellingham Bay rail line near a junction with a Northern Pacific line that extended north to Sumas on the border.
In 1906-7, the White City Amusement Park was built in the Silver Beach area and operated until 1919.
By 1918, the ancient trees closest to the lake were gone, and logging camps moved east into the hills.
Early in the 20th century, swimming was discouraged or prohibited because of water contamination concerns.
By the 1930s, the lake had become a popular recreation area with many summer cottages. Boating, swimming and fishing became the order of the day.
Lumber mills at the north end of the lake shut down in the 1940s and timber magnate J.H. Bloedel donated the land that became Bloedel Donovan Park.
Fears that too much development would harm the lake became reality in 1998 when a reduction in dissolved oxygen in the lake's depths prompted the state Department of Ecology to put Lake Whatcom on its list of impaired water bodies. Research since then has made it clear that the oxygen problem is related to phosphorus pollution caused by runoff from developed areas.
Local governments are now under a state mandate to develop plans for major reductions in phosphorus runoff into the lake. So far, efforts to curb phosphorus contamination have not succeeded; phosphorus levels in the lake continue to rise slowly.
Phosphorus is superfood for algae that die and become food for bacteria. The bacteria deplete dissolved oxygen and make the lake less hospitable to fish.
Algae also have clogged city water filtration systems at times. In 2009, the problem reached crisis levels and the city imposed water rationing because of reduced flow through the system.
The city now charges homeowners $12 a month on their water bills to raise millions for the purchase of undeveloped real estate around the lake, hoping to head off worse problems.
In late 2011, biologists found that the non-native Asian clam had found its way into the lake, touching off fears that destructive zebra and quagga mussels could also gain a foothold. Mandatory boat inspections are expected to be in place by spring 2013 in the hope of heading off any more invasions.
Children younger than 6 and women of childbearing age are advised to not eat bass from the lake because of mercury contamination that appears to be due to natural sediment concentrations, pollution from global sources, plus additional mercury from the Georgia-Pacific Corp. chlorine plant and municipal waste incinerators, which have been shut down for years.