Gorunna Trench

A really dark, foreboding sea trench (even krakens only go there in pairs) mentioned in The Colour of Magic

It is believed that some ancient horrors from the Dungeon Dimensions lurk in the depths of the Gorunna Trench, left over from the time when they used to walk the disc. There appears to be some truth in this story; once a pirate captain had the misfortune of taking the Luggage aboard his ship and then threatened it's owner. After it had eaten half his crew, the captain chose to scuttle his ship with the Luggage, while he and his men escaped on a raft.

The ship sank down into the dark reaches of the Gorunna Trench, until something ate it up in one gulp.

Some days later natives on a small island close by where amazed to find the dead body of a 'vast sea monster' washed up on their beach. As far as they could tell, it appeared to have been trampled to death.

Annotation

The Roundworld reference is to the Marianas Trench, the deepest trench on Earth (so deep in fact, that if you put Mt Everest into it, there would still be a mile of water above it!).