how can I say that there is some hidden 'pit'(?) in my bed? I want to express that the 'pit' is under the level of the flat bed surface. How can I say that? For my understanding 'pit' is something I dig with a shovel. I would like to know a word I can use not only referring to bed or furniture but just to be able to say that there is some 'blabla' (valley?,LOL, but not about nature) within a flat surface, so it goes flat, flat, flat then deeper for a while and then flat again. Thanks.
I think what you're describing is a storage compartment which, in terms of furniture, would typically be referred to as a "hidden compartment" or "secret compartment" or, depending on the way it's accessed, a "secret drawer."
Those terms wouldn't be relevant to all flat surfaces, but the following nouns are some generic options for describing changes in a surface:
A thesaurus would provide additional options. You could use them interchangeably in a sentence like:
The surface was flat except for a dip in the center.
Seems like I've found the word, so it's " hollow"
Definition of hollow (Entry 1 of 4) 1 : an unfilled space : CAVITY, HOLE in the hollow of a tree 2 : a depressed or low part of a surface especially : a small valley or basin https://www.merriam-webster.com/dictionary/hollow