Anne Helmenstine
The longest black sand beach on the island of Hawaii is in Waipio Valley, which you can reach by four wheel drive, walking, or horseback. The surf is awesome, but treacherous.
This black sand beach is a mile long, though it is not easy to access. The tsunami warning system is located at this lookout also. I was told a four hour evacuation warning is available for residents and visitors to this valley. Waipio Valley is sometimes called the Valley of the Kings.