Definitions

1. adjective

relating to or designating the deepest parts of the ocean, especially the zones of near-total darkness and extreme pressure near the bottom of the deep sea

“The hadal zone is home to unique and rare species of fish and invertebrates.”

2. noun

the deepest part of the ocean, especially the zone of near-total darkness and extreme pressure near the bottom of the deep sea

“The hadal trenches are some of the most unexplored regions on Earth.”