To say it was complete would actually be an overstatement.
For Prehistoric World not only had the land, but also the Heavenly Realm and the Netherworld. Therefore, the so-called completion of the water veins of Prehistoric World was merely the completion of the terrestrial water systems—those of the Heavenly Realm and the Netherworld were still left out.
The true grand completion of the water veins of Prehistoric World would mean the unification of the waters of the Heavenly River above, the rivers and seas on the Earth, and the Underworld River and Netherworld River below, connecting all water systems within and outside the Three Realms to ensure unobstructed flow; only then would the Three Realms be truly integrated, and the water veins between Heaven and Earth could achieve genuine completion.
