'Beijing Dabaotai Museum of the Western Han Mausoleum' is the tomb of Liu Jian (73-45BC), a king of the Western Han dynasty. It was an underground palace, with inner coffins inside outer, inside chambers, inside surrounding corridors - with three chariots and eleven horses in the entrance passageway. It was built with astonishing quantities of cypress timber (deforestation started early in China), carefully sealed with layers of white clay and charcoal.
The museum is next to Beijing World Park in Fengtai district. Not many people seem to have heard of it, so for ease of finding, here is the info from the ticket, with the name of the museum (the first few characters) and a map. Tel 6373 6427 was correct as of March 2001. Opening hours are 9-4 Mon-Thurs, 9.30-4 Fri-Sun.