A situation or problem is so complex and involves so many intricate details or historical background that one doesn't know where to begin explaining it; to be overwhelmed by the sheer volume or intricacy of information.
这个项目的进展情况，真是“一部二十四史，不知从何说起”，牵扯到的部门和环节太多了。
The progress of this project is truly a case of "a volume of Twenty-Four Histories, I don't know where to begin," as it involves too many departments and steps.
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