Murilo is a large atoll formation located within the central Caroline Islands region; together with the neighbouring atoll of Nomwin it forms the Hall Islands group of northeastern Chuuk State. Chuuk lies 101 km to the south-southwest and immediately to the southwest is the large atoll of Nomwin, 9 km distant.

Geologically both the Nomwin and Murilo atolls are emergent parts of the same large, elongate seamount with Murilo occupying the northeastern section and Nomwin the southwest (see panoramic image). Such double atoll formations are quite rare, with other examples including the two atolls of Elato, and the Ahe-Manihi and Takapoto-Takaroa seamounts of the Tuamotu Archipelago.

Murilo has a maximum length of 38 km (northeast to southwest); its greatest widths are found in the northeast (19 km across), narrowing towards the southwest (4 km across). The narrow reef rim, enclosing a deep lagoon of some 350 km² in area, is sunken along much of the southern side of the atoll, while the northern rim is more pronounced. Upon the rim are 28 small islets with a combined land area of 1.3 km².

image: earth sciences and image analysis laboratory, nasa johnson space center