Learn Dutch: Het grote anker ligt helemaal beneden. - The big anchor lies all the way downstairs.

Beneden describes a fixed location ("downstairs" or "at the bottom"). You would use naar beneden if there was motion towards that place, like bringing the anchor "down". Because the anchor is just resting there (indicated by the verb ligt), we use the fixed location adverb.

Could I say onder instead of beneden to mean "below"?

No, because onder usually acts as a preposition meaning "under" something specific (like onder de tafel for "under the table"). Beneden is an absolute spatial adverb, meaning it stands alone to describe the general space "down below" or "downstairs".

What is the role of helemaal in this sentence?

While helemaal often means "completely," when it is paired with spatial adverbs like beneden or boven, it translates to "all the way." So helemaal beneden means "all the way downstairs" or "at the very bottom."

Why is the verb ligt used instead of just saying "is"?