moon pool

Etymology
From, from the fact that, on some nights, moonlight can be seen reflected by the surface of the sea through the opening, making it resemble a swimming pool.

Noun

 * 1)  An opening in the base of the hull of a vessel (especially a marine drilling platform), which is used for lowering equipment into the sea below.

Translations

 * Dutch: moonpool
 * French:
 * German: Moonpool
 * Japanese: ムーンプール
 * Swedish: månbassäng