The Line Islands comprise a lengthy and loose group of low-lying, extremely remote coral islands, atolls and reefs straddling the equator in the central Pacific Ocean. The chain, stretching for over 2,000 km from north to south, has eleven islands with a combined land area of around 425 km², with most of this total being contributed to by the large atoll of Kiritimati at 321 km². Eight of the islands — Flint, Kiritimati, Malden, Millennium Atoll, Tabuaeran, Teraina, Starbuck and Vostok — are part of the Republic of Kiribati. Jarvis Island, Kingman Reef and Palmyra are uninhabited, unincorporated territories of the United States, managed as part of the Pacific/Remote Islands National Wildlife Refuge complex.
The group forms one of the remotest island chains on Earth: the Rawaki Islands (also forming part of the nation of Kiribati) lie 1,800 km to the west, the Hawaiian Islands lie 1,600 km to the north and the Marquesas, Tuamotu Archipelago and Society Islands to the southeast and south. The atoll of Penrhyn in the northern Cook Islands is the Line Islands nearest central Polynesian neighbour, lying 610 km northwest from Vostok.
The Line Islands are sometimes divided into three groups: the Northern, Central, and Southern Line Islands; only the northern islands of Kiritimati, Tabuaeran and Teraina are inhabited. These remote communities are located around 2,600 km east of the Kiribati capital on Tarawa in the Gilbert Islands. The more isolated islands of the central and southern groups tend to have drier climates than those in the north and are all uninhabited, being preserved as strict nature reserves.
Geologically, the Line Islands are one of the major seamount chains to be found in the Pacific. The distant Johnstoll Atoll is also considered to be part of Line Islands seamount chain, giving the group a total length in excess of 4,000 km. The geological history of the Line Islands seamount chain is complex and poorly understood. Volcanic in origin, the islands are thought to have formed over a series of hot-spots — up-wellings of magma from deep within the Earth — acting at weaknesses in this part of the ocean floor. The last volcanic activity in the Line Islands occurred around 37 million years ago in the centre of the group. Since then, sinking of the sea bed, changing sea levels and the forces of erosion have acted on these seamounts so that now only 11 emerge, at best, a few meters above sea level.
Even though the islands exhibit low levels of endemism and biodiversity, all of the islands in the chain are considered as internationally important for their large populations of breeding seabirds — particularly Kiritimati, Malden, Millennium and Starbuck. Seabird colonies (23 breeding species) in the Line Islands are among the world's largest.