井底 is most famous from
井底之蛙 (jǐngdǐ zhī wā, "the frog at the bottom of the well") — a classic figure for someone with a narrow, parochial view of the world, originally from 《
庄子·
秋水》: the well-frog can only see the patch of sky framed by the well's mouth and assumes that's all the sky there is.
