Synonyms for the phrase "sink into" include "plunge," "immerse," "submerge," "drown," and "engulf." These words all convey a sense of being fully enveloped or absorbed in something. "Plunge" and "immerse" can also suggest a sudden, intense action, while "submerge" and "drown" imply a more complete covering or suffocation. "Engulf" is often used to describe a situation where something or someone is overwhelmed or engulfed by circumstances or emotions. Overall, these synonyms for "sink into" convey a sense of being overtaken or consumed by something, whether it's physical or emotional.