1. too deep to measure or reach the bottom of; extremely deep
Usage: literal use, often nautical or geographic
2. impossible to understand or comprehend; mysterious or inscrutable
Usage: figurative use

Examples

• The ocean's fathomless depths remain largely unexplored.
• She stared into his eyes with a fathomless expression.
• The ancient text contained fathomless mysteries that scholars still debate.
• The well appeared fathomless, dropping far beyond the reach of any rope.
• His fathomless wisdom seemed to come from years of experience.
• The canyon's fathomless gorge stretched endlessly below us.