Menehune Ditch, Kauai
Although this site is not well developed and is seldom visited by tourists, I found it absolutely fascinating. The Menehune Ditch is an aqueduct of cut and dressed stones that was once used to bring irrigation water from the Waimea River to the taro fields. Only 50 or 60 feet of it are now visible, including a section of tunnel. The rocks for construction of the ditch came from a quarry more than six miles away.
What's so impressive about is that this is one of the only relics that was almost certainly NOT created by the current race of Hawaiians who came from Tahiti around 1,000 years ago. It was probably built by the original "native Hawaiians", who migrated from the Marquesas Islands around 300 A.D. The Tahitian invaders displaced them and their culture after they had lived on the islands peacefully for 700 years, establishing the Hawaiian culture that was discovered by Captain Cook, the first European to "discover" these islands in the middle of the Pacific Ocean.
On Menehune Road, 1.3 miles north of the town of Waimea. Look for the ditch directly across the narrow roadway from the Waimea Swinging Bridge, which you can't miss.
As you enter the town of Waimea from east, turn right onto Menehune Road which follows Waimea River upstream into the canyon. Soon you'll come across a suspension footbridge on your right; on your left you will find a water ditch that parallels the road.
Apparently unremarkable - were it not for the fact that construction of the ditch seems to predate the arrival of the current Polynesian-Hawaiians. The ditch is built out of cut stones, a construction technology the Hawaiians did not possess. The Hawaiian people attribute this and other such stoneworks throughout the islands to the Menehune - a Hobbit-like race with supernatural ability to build things who lived on the islands before the Hawaiians arrived.