The mouth of a well will surround you if you fall into it. The mouth of a well is the opening of the well, and if you fall into that well, the mouth as well as the rest of the well will surround you.
The well walls surround you as you look up to the mouth once again. This time you see an eagle (い) flying overhead. You call out to the eagle to save you and the eagle comes swooping down into the well and flies you out to safety. What a nice eagle!