A city overrun with outsiders, Venice manages every two years to squeeze in another legion or two for the Venice Biennale, the art world's enduring multinational group grope. Two years ago, the U.S. pavilion devoted itself to the venerable American conceptualist Bruce Nauman, who walked away with the top prize, the Golden Lion. This year the pavilion has been offered to a pair of artists whose names are not nearly so well known, Jennifer Allora and Guillermo Calzadilla, who work in sculpture, photography, video and performance art with a political edge. The materials for their Biennale debut include an upside-down tank with a treadmill on top, a pipe organ fitted with a functioning ATM, and a small squadron of world-champion gymnasts. As Stephen Sondheim once put it, "Art isn't easy." 6/4–11/27