1. impossible to understand or comprehend
2. too deep to measure
Usage: literal sense, often nautical

Examples

• The complexity of the human brain remains unfathomable to scientists.
• Her decision to quit her dream job was unfathomable to her friends.
• The ancient ocean depths were unfathomable to early explorers.
• His unfathomable wisdom impressed everyone who met him.
• The tragedy left the community with unfathomable grief.
• The lake appeared unfathomable in the moonlight.
• She stared into the unfathomable darkness of the cave.