Here's why:
* Gradable antonyms are words that express opposite ends of a spectrum or scale. For example, "hot" and "cold" are gradable antonyms because there are varying degrees of hotness and coldness.
* Shallow and deep are both words that describe depth, and they are gradable because something can be very shallow, moderately shallow, or very deep.
Other possible gradable antonyms for "shallow" might include:
* Profound
* Substantial
* Meaningful
However, these words are more related to the meaning or significance of something, rather than its literal physical depth.