The phrase "grew thick" refers to something that has become dense or concentrated. This can be used to describe physical objects like plants or forests, as well as more abstract concepts like tension or emotion. Synonyms for "grew thick" include flourished, proliferated, thrived, flourished, crowded, teemed, multiplied, swelled, and thickened. "Flourished" suggests a strong and healthy growth, while "proliferated" or "multiplied" both imply an increase in quantity. "Thickened" implies a denser consistency, while "crowded" and "teemed" suggest a lot of activity or movement. These synonyms can be used interchangeably to describe a wide range of situations where something has become more dense or concentrated over time.