Minerva reef is an atoll that has completely eroded to the surface of the ocean, leaving an inner harbor of calm in the middle of the South Pacific. Apparantly it is a spectacular site to be safely anchored in the middle and see the ocean waves crashing against the reef 360 degrees.
A favorite stop for sailboats transiting from Tonga to NZ it is covered in shipwrecks.
A great tale of survival is found here the story of a Japanese fishing boat which wrecked there in 1959.