New evidence suggests that the Great Circle’s ditch was designed to hold water; ancient ditches and ponds at some other earthworks still do. Dr. Brad Lepper suggests what this may have meant:

I think it was intended to evoke in some way, or presence in some way, that watery underworld of Native American traditions: the Beneath World. And whether it’s in the form of a spirit barrier, or whether it’s just simply to have that presence of water surrounding the ceremonies… that’s probably as close as we can come to understanding the purpose now.

The ancient clay and limestone lining of the ditch probably held water for centuries, but in the long run would have required maintenance. This explains why we don’t see water in it today:

Over the centuries, as roots penetrate that, the seal is penetrated. And the Newark Earthworks are built on hundreds of feet of glacial sand and gravel. So you could be pouring water into that all day, and unless it was sealed that water would just vanish.

The Great Circle’s inner ditch was originally constructed to hold water, making a continuous reflecting pool or “spirit barrier”.