Simmering down is a phrase used to describe the calming or quieting of a situation or emotion. However, there are several antonyms that can be used instead of simmering down to describe situations or emotions that are escalating. Some of these antonyms include boiling over, exploding, flaring up, getting out of control, and raging. These words are used to describe volatile situations where emotions are running high or conflicts are reaching a breaking point. Unlike simmering down, which is a positive phrase describing a decrease in intensity, these antonyms have negative connotations, describing situations where things are spiraling out of control.