Question: In what ocean is the island in which were found subfossil remains of the Henderson ground dove? 

Context paragraphs:
Plain-breasted ground dove: The plain-breasted ground dove ("Columbina minuta") is a species of bird in the family Columbidae.  They lack the scaled appearance to the feathers of the similar and typically more abundant common ground dove.  With a total length of approximately 14–16 cm (5.5-6.3 in) and a weight of 24 - , this species averages slightly smaller than the common ground dove.  The plain-breasted ground dove may be the smallest pigeons or doves by mass, though the more heavily built but almost tailess dwarf fruit dove of Southeast Asia is typically shorter in overall length.

Tongan ground dove: The Tongan ground dove ("Alopecoenas stairi"), also known as the shy ground dove, is a species of bird in the family Columbidae.  The species was formerly known also as the friendly ground dove.

White-breasted ground dove: The white-breasted ground dove, white-bibbed ground dove, or purple ground dove ("Alopecoenas jobiensis"), is a species of bird in the family Columbidae.  It is found in New Guinea, the Bismarck Archipelago and the Solomon Islands.  Its natural habitats are subtropical or tropical moist lowland forests and subtropical or tropical moist montane forests.

Babyrousa bolabatuensis: The Bola Batu babirusa ("Babyrousa bolabatuensis") is a proposed species of babirusa from the Indonesian island of Sulawesi.  It was first described in 1950 as a subspecies of "Babyrousa babyrussa", then the only recognized species of babirusa, and raised to species rank by Colin Groves and Erik Meijaard in 2002.  At present the Bola Batu babirusa is only known for certain from subfossil remains from the southern arm of Sulawesi.  Based on a single skull from central Sulawesi it has been suggested that babirusas from this part of Sulawesi represent an extant population of the Bola Batu babirusa, and this was followed in the third edition of "Mammal Species of the World".  However, the most recent major review also found similarities between the central Sulawesi specimen and the Togian babirusa, leading them to conclude that it represents an undescribed taxon and that the taxonomic position of central Sulawesi babirusas only can be determined through additional specimens.  Subfossil remains from the south-western arm of Sulawesi, where now likely extinct, have been classified as Bola Batu babirusas, but these were considered unclassifiable in 2002, as were extant populations from the eastern arm of Sulawesi and Buton due to the lack of specimens.  Due to these uncertainties, the IUCN Red List provisionally synonymized "B. bolabatuensis" under the northern Sulawesi species, "B. celebensis", pending clarification of the taxonomy of Sulawesi babirusas.

Henderson archaic pigeon: The Henderson archaic pigeon ("Bountyphaps obsoleta"), or Henderson Island archaic pigeon, is an extinct bird in the Columbidae, or pigeon family.  It was described in 2008 as a new genus and species from subfossil remains found on Henderson Island in the Pitcairn Group of south-eastern Polynesia.

Sulawesi ground dove: The Sulawesi ground dove ("Gallicolumba tristigmata") also known as yellow-breasted ground dove is a medium-sized, approximately 35 cm long, olive-brown ground dove with golden forehead, yellow breast, red legs, iridescence bluish-green crown ended with purple patch behind ear coverts, dark brown tail and white below.  Both sexes are similar.

Henderson imperial pigeon: The Henderson imperial pigeon ("Ducula harrisoni"), or Henderson Island imperial pigeon, is an extinct species of bird in the Columbidae, or pigeon family.  It was described from subfossil remains found on Henderson Island in the Pitcairn Group of south-eastern Polynesia.

Pitcairn Islands: The Pitcairn Islands ( ; Pitkern: "Pitkern Ailen "), officially Pitcairn, Henderson, Ducie and Oeno Islands, are a group of four volcanic islands in the southern Pacific Ocean that form the last British Overseas Territory in the Pacific.  The four islandsPitcairn, Henderson, Ducie, and Oenoare spread over several hundred miles of ocean and have a total land area of about 47 sqkm .  Only Pitcairn, the second-largest island that measures about 3.6 km from east to west, is inhabited.

Henderson ground dove: The Henderson ground dove ("Alopecoenas leonpascoi"), or Henderson Island ground dove, is an extinct species of bird in the family Columbidae.  It was described from subfossil remains found on Henderson Island in the Pitcairn Group in 1991.  Its relatively small wings suggest it was flightless.

Subfossil lemur: Subfossil lemurs are lemurs from Madagascar that are represented by recent (subfossil) remains dating from nearly 26,000 years ago (from the late Pleistocene until the Holocene) to approximately 560 years ago.  They include both living and extinct species, although the term more frequently refers to the extinct giant lemurs.  The diversity of subfossil lemur communities was greater than that of present-day lemur communities, ranging from as high as 20 or more species per location, compared with 10 to 12 species today.  Extinct species are estimated to have ranged in size from slightly over 10 kg to roughly 160 kg .  Even the subfossil remains of living species are larger and more robust than the skeletal remains of modern specimens.  The subfossil sites found around most of the island demonstrate that most giant lemurs had wide distributions and that ranges of living species have contracted significantly since the arrival of humans.

Supporting paragraphs: Henderson ground dove Pitcairn Islands
Answer: Pacific Ocean