The phrase "solid as rock" implies a great degree of strength, stability and durability. However, there are a number of antonyms for this expression that denote weakness, instability and vulnerability. For example, "fragile as glass" could be an antonym for "solid as rock," showing that something is prone to breaking or being easily destroyed. "Unstable as sand" might also work, suggesting that an object or situation is flexible or unsteady. "Flimsy as paper" could be another antonym, as it implies that something is insubstantial or easily torn. Ultimately, the antonyms of "solid as rock" all reflect a fragility and weakness that the original phrase does not possess.