The phrase "bottomless depths" is often used to describe a vast and seemingly endless expanse, such as the ocean. However, there are several synonyms that can be used to convey a similar sense of depth or immensity. One option is "abyss", which typically refers to a deep, dark chasm or hole. Another potential synonym is "fathomless", which suggests a depth that is impossible to measure or fathom. "Infinite expanse" or "endless void" could also be used to convey the vastness and unbounded nature of a deep space or other abyss-like environment. Ultimately, the choice of synonym will depend on the specific context and intended tone of the message being communicated.