moon pool

noun

(in the oil industry) an open shaft in the centre of the hull of a ship engaged in deep-sea drilling through which drilling takes place

Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012