In this three dimensional physical world, the only difference between height & depth is one of perspective.
I look up at the heights and I look down into the depths.
The physical world is measured in three separate directions of length, width or breadth & height or depth.
Because all physical objects have a length, breadth & height, we believe that we live in a three dimensional world.
Yet the physical world also has great depth.
Length, breadth & height is measured in imperial miles, yards, feet & inches or in metric kilometres, metres, centimetres & millimetres.
Depth is measured in Holons.
Depth is an order of increasing holism or wholeness.
Looking deep into the microcosm: I can measure the depth of an atom contained in a molecule that is contained in a cell, contained within an organ, that is contained within a human body.
Looking deep into the macrocosm: I can measure the depth of a human body that is contained within the biosphere of earth, that is contained within the solar system of our sun, that is contained within the galaxy that is our milky way, that is contained within our physical Universe.
The Universe that is our Cosmos is a holarchy of holons with increasing depth & wholeness.