The find at the World Heritage site has been described by noted archaeologist Matthew Stout as “the most definite, complex, and ordered feature ever seen in a geophysical survey in Ireland”.
Mr Stout is leading the dig alongside his wife, Geraldine.
The four-week excavations started last week and it may be another week before the team of 18 archaeologists uncover anything of real significance.
A survey was conducted in 2015 by Joanna Leigh to investigate the feasibility of renovating the old tourist office at Newgrange and clearly identified the substantial site at the back of Newgrange Farm.
It suggests numerous large pits, forming two parallel sets extending over 75m, indicating an ancient processional way to Newgrange.
The entrance way and perpendicular pits form a passage or corridor, which ends precisely due south of the entrance of the Newgrange Passage tomb.
Ms Leigh has speculated that the site is therefore associated with the passage tombs and is ritual in nature.
“It’s the most impressive geophysical underground site that I’ve ever seen,” said Mr Stout after the first week of excavations.
“It shows four even rows of pits and ditches and is an extraordinary feature.”