It is roughly triangular in shape, with the base running 82 kilometres (51 miles) along its southern side.

The southwest corner of the triangle is marked by Ulul islet, the main island (largest and most populated), also called Onoun.

The reef rim of the atoll is submerged for the greater part and is marked by the lighter colour of the water over it.

The atoll is home to five of the 40 municipalities of Chuuk state, listed in counterclockwise sequence, with population (2000): Namonuito was first visited by Spanish explorers on the galleon San Jerónimo on 5 July 1566.

[1][2] Onou Island was later visited by Spanish naval officer Juan Antonio de Ibargoitia commanding the vessel Filipino in 1801 who charted it as Anonima (Anonymous).