Ocean drops of up to below the present level during Pleistocene glacial periods revealed the vast lowlands known as Sundaland, enabling hunter-gatherer populations to freely access insular Southeast Asia via extensive terrestrial corridors. Modern human presence in the Niah cave on East Malaysia dates back to 40,000 years BP, although archaeological documentation of the early settlement period suggests only brief occupation phases. However, author Charles Higham argues that, despite glacial periods modern humans were able to cross the sea barrier beyond Java and Timor, who around 45,000 years ago left traces in the Ivane Valley in eastern New Guinea "at an altitude of exploiting yams and pandanus, hunting and making stone tools between 43,000 and 49,000 years ago."