A collection of fossil bones, some dating back 40,000 years, jut out of the natural asphalt at Tar Pit 91, the only active urban excavation site in the world at the Page Museum at the La Brea Tar Pits in Los Angeles. It remains open for two months a year, when scientists and volunteers descend 4.5-metre underground to remove, clean and catalogue a baffling array of Ice Age biodiversity. This season it will be open from June 25 to September 11. The abyss is tucked between world-class art museums, high-rise condominiums and some of the busiest automobile traffic in the US.