having fallen to the bottom of the sea

They're diving for sunken treasure. 他们潜入海中寻找沉没的宝藏。

at a lower level than the surrounding area

It was a luxurious bathroom, with a sunken bath. 那是一个豪华的浴室，里面有一个下沉式浴缸。

(of eyes or cheeks) seeming to have fallen further into the face, especially because of tiredness, illness, or old age

She looked old and thin with sunken cheeks and hollow eyes. 她脸颊凹陷，目光空洞，看起来苍老瘦削。