Abyssal
Part of speech: adjective
Definition: Belonging to, or resembling, an abyss; unfathomable. Of or belonging to the ocean depths, esp. below 2000 metres (6500 ft): abyssal zone
Unfathomable
Part of speech: adjective
Definition: impossible to fathom or understand; incomprehensibledifficult to penetrate.
Example sentence: It's sad to see such institutions as 'All My Children' and some of the others, like 'Guiding Light,' which have been on the air for, like, 40 or 50 years. It's almost unfathomable to see that they actually aren't going to be on the air anymore. It's really sad.