When you want to get to the bottom of something, you may use the phrase "root the matter". However, there are other synonyms you can use to convey the same message. For example, you can say "uncover the truth", "get to the crux of the issue", or "dig deeper". Another phrase that works well is "find the source of the problem". If you want to use a single word instead, you can opt for "investigate", "determine", or "ascertain". Regardless of the phrase or word you choose, the end goal is always to understand the matter fully and resolve any underlying issues.