"Now what I am commanding you today is not too difficult for you or beyond your reach. It is not up in heaven, so that you have to ask, "Who will ascend into heaven to get it and proclaim it to us so we may obey it? Nor is it beyond the sea, so that you have to ask, "Who will cross the sea to get it and proclaim it to us so we may obey it? No the word is very near you; it is in your mouth and in your heart so you may obey it."
In our limited 3-D human thinking we cannot see heaven so we use the term 'up'. God lives up there above the sky and clouds, because it is beyond the reach of our physical eyes and it is a dimension that we understand because we have experienced it in our 3-D experiences.
This scripture is actually guiding us toward realizing that God and his heavenly invivsibly kingdom are not actually up, but very near us even in within us. He is speaking in 5th, 6th, 7th dimension terms. He is showing us a deeper truth than we can comfortably express in 3-D terms. The Bible addressed the terms we are comfortable with- up (height) and across (length or width) and then in effect tells us that Heaven is more than these dimensions- it is not limited to these! "No, the word is very near you....in your heart". The no is pointing out something greater than the up and across dimensions. The invisible realm penetrates all that we see - even our own bodies which are mere tents! Tents have space or dimensions within them- they are not solid within their canvas walls.
A dimensional point to ponder and marvel at the depth of our world and the world to come.