Trying to determine which U.S. lake is the cleanest is a nearly impossible taskthere are hundreds of variables and no official databases. But we decided to give it a shot. And the crown goes to Oregon's CRATER LAKE. Our reasons? For one thing, there's the water clarity. On its best days, 1,943-foot-deep Crater is as clear as a shot of Tanqueray: You can peer down 142 feet into its blue depths. And since there are no tributaries flowing into or out of the 13,760-acre basinwhich is fed almost exclusively by the 533 inches of snow caught by its namesake crater each yearlittle sediment or contamination gets in. Added bonus: The lake's remote location, in southwest Oregon, keeps weekend warriors away from this national park. You won't find jet skis here; only six boatsfour tour ferries and two research vesselsare allowed on the water. Visitors can hike down from the crater rim to the shore for an icy dip (the lake hovers around 50 degrees in the summer), but the best way to experience the lake is to find a warm rock overlooking the water and let the view clear your mind.