Nelsons Island occupies the only emerging reef structure in the northern fringe of the Great Chagos Bank.

The island stretches in an East-West direction, with a length of 1.6 km (1 mi) and 250 m (820 ft) wide in its broadest area.

In terms of its geology, detailed photographic images show evidence of a coral limestone core that has been overlaid by sandy loam.

[6] The island is unique for the Chagos for having only two species of tree, Coconut and Pisonia, which both appear to have arrived naturally.

[4] One of the major problems affecting the native wildlife is the significant accumulation of plastic bottles and other assorted debris such as damaged fishnets, shoes as well as an assortment of plastic and polystyrene materials that are washed up by localised ocean currents, winds and sea-storms.