Many historical events remain undocumented due to lack of written records.

undocumented 是由前缀 un-（表示"否定"）和 documented（"有记录的"）组成的形容词。documented 来自动词 document（记录、记载），加上 -ed 变成形容词形式。undocumented 整体意思是"未被记录的、无书面证明的"。