What was placed over the mouth of the lions' den after Daniel was thrown in?

Answer: A stone

A stone was placed over the mouth of the den, and the king sealed it with his own signet ring and the rings of his nobles.

“A stone was brought and placed over the mouth of the den, and the king sealed it with his own signet ring and with the rings of his nobles, so that nothing concerning Daniel could be changed.”— Daniel 6:17 (BSB)