Unfathomable means impossible to ever understand. For most people, the field of quantum mechanics is unfathomable.
Fathoms are seafaring units of measure equaling about six feet. So something that is unfathomable is also immeasurable, especially when it comes to depth. If your sonar isn't bouncing anything back to you, you're dealing with an unfathomable distance. Metaphorically, unfathomable can refer to something that resembles an abyss, some figurative notion that's so deep you can't even measure it.
adj of depth; not capable of being sounded or measured
having great spatial extension or penetration downward or inward from an outer surface or backward or laterally or outward from a center; sometimes used in combination
too deep to determine the depth of