The coastal area is flat with good aerial approaches, and therefore suitable for airstrips, although it is intercut by many tributaries of rivers and mangrove swamps. Owing to the swampy lands and high rainfall, about per year, the area is prone to malaria and flooding. After floods, the coastal plains become "virtually impassable quagmires of glutinous mud", and the ground is not suited for development. The bay is bounded to its north and south by the Stirling Ranges, which at points rise to and are covered in Kunai grass and dense scrubland. The main area of firm ground suitable for construction and development is found directly at the head of the bay. In 1942 this area was occupied by plantations of palm oil, coconuts and cocoa, as well as a number of jetties and villages, connected by what was described by Major Sydney Elliott-Smith of the Australian New Guinea Administrative Unit (ANGAU) as a "modest 'road' system" that was, in actuality, only a dirt track wide. The area was sparsely populated, although there were a number of villages along the track. Ahioma was situated the farthest east, and together with Gili Gili in the west, it bounded Lilihoa, Waga Waga, Goroni, KB Mission, Rabi and Kilarbo. The Japanese thrust into the Pacific region had begun in early December 1941 with attacks against British and Commonwealth forces in the Battle of Hong Kong and the Malayan campaign, and against the US Pacific Fleet, much of which was caught at anchor in Pearl Harbor. They rapidly advanced south, overwhelming resistance in Malaya, capturing Singapore in February 1942, and successfully occupying Timor, Rabaul and the Dutch East Indies. While a Japanese naval operation aimed at capturing Port Moresby was defeated in the Battle of the Coral Sea in May, elsewhere American forces in the Philippines capitulated, and Japanese forces advanced towards India through Burma. Although the Japanese had been defeated in the Coral Sea, another attempt at capturing Port Moresby was anticipated. The Allied Supreme Commander of the South West Pacific Area, General Douglas MacArthur, decided to establish airbases to protect Port Moresby. To the west, he authorised the construction of an airbase at Merauke in Netherlands New Guinea. Another, codenamed "Boston" was authorised to the east in the largely unexplored Abau–Mullins Harbour area on 20 May. Any Japanese force approaching Port Moresby by sea would have to sail past these bases, allowing them to be detected and attacked earlier; but the base in the east had other advantages too.