Apparently, it’s a type of spider called a Cyclocosmia. Wikipedia sez:
The abdomen of spiders in this genus is abruptly truncated and ends in a hardened disc which is strengthened by a system of ribs and grooves. They use this to clog the entrance of their 7 to 15 cm deep vertical burrows when threatened, a phenomenon called phragmosis. Strong spines are located around the edge of the disc. The four spinnerets are found just anterior to it, with the posterior, retractable spinnerets particularly large. C. ricketti females are 28 mm long, with a disc diameter of 16 mm. Only the bottom portion of the burrow is silk lined.