There is something quietly miraculous about The London Mastaba, Christo and Jeanne-Claude’s 600-ton pyramid-like form that glows red-orange and seemingly sprouts out of its own watery reflections in London’s Serpentine Lake. London is full of verticals, but the glistening structure (on view through September 23) is a matter of horizontals. Rows of 55-gallon barrels laid end to end comprise 90-foot-long flanks sloping at a 60-degree angle, each cylindrical container colored with identical bands of red and white. The rows are minutely off-kilter; the alternating bands that result create a kind of Op Art shimmer.