LONDON — Wearing a hard hat and a cargo jacket, the artist Christo stood on a platform looking over the Serpentine lake one April morning and watched his latest creation come to life. As ducks glided across the water, men in orange jumpsuits began assembling the installation, a crane hovering above their heads.

“The London Mastaba,” Christo’s first major outdoor work in Britain, is now floating (through Sept. 23) in the middle of the lake in Hyde Park. A trapezoidal pyramid of 7,506 painted and horizontally stacked barrels, it’s 66 feet tall — as tall as the Sphinx in Egypt — and weighs roughly 650 tons. Named after a flat-roofed structure with sloping sides that originated some 6,000 years ago in Mesopotamia (the word “mastaba” means “bench” in Arabic), it’s a test for a mastaba roughly eight times as high that Christo hopes to put up in the desert in Abu Dhabi.