“Full fathom five my father lies”
is a quotation from a well-known play by William Shakespeare about a storm, featuring Caliban,  Ariel et al.

 Photo credit Wikipedia

A fathom is a measure of depth in water. Five fathoms equal thirty feet.  A rope with knots tied at equal distances (fathoms?) along it is lowered with a weight attached.  Calculation of depth is straightforward by this method.

Hard to fathom
means difficult to understand.

There is a similarity with plumbing depths.

This gives an impression of going into something in detail.  Plumb comes from a Latin word meaning lead, a heavy metallic element.

Depth charges by contrast either explode like limpet mines or relieve constipation like syrup of figs.

“Going down” is an expression used by British lift or American elevator operators in times past.  Something that goes down well is enjoyable or well-received.  I hope this post will be both.

Psalm 107 is worth reading.  It is a reflection on depths of despair, which ends by rising above them because of divine lovingkindness.

Posted in response to Daily prompt: three letter words