The occurrence of mammals of the Marsupial order in the Molucca Islands and Celebes, while none have been found in the adjacent islands of Java and Borneo, lying on the west of Wallace's line, or in the Indian region, shows that the margin of the Australian region has here been reached.
The China Sea on the north has a maximum depth of 2715 fathoms off the Philippines, the Sulu Basin reaches 2550 fathoms, and the Celebes Basin 2795 fathoms. Some of the channels between the islands are of very great depth, Macassar Strait exceeding 1000 fathoms, the Molucca Passage exceeding 2000 fathoms, and the Halmahera Trough sinking as deep as 2575 fathoms. The deepest of all is the Banda Basin, a large area of which lies below 2500 fathoms and reaches 3557 fathomsin the Kei Trench.
The islands are often described as of two groups, Java and Madura forming one, and the other consisting of Sumatra, Borneo, Riouw-Lingga Archipelago, Banka, Billiton, Celebes, Molucca Archipelago, the small Sunda Islands, and a part of New Guinea-the Outposts as they are collectively named.
Borneo began to be known to Europeans after the fall of Malacca in 1511, when Alphonso d'Albuquerque despatched Antonio d'Abreu with three ships in search of the Molucca or Spice Islands with instructions to establish friendly relations with all the native states that he might encounter on his way.
The Molucca Islands being, at that time, the principal objective of European traders, and the route followed by - Magellan's ships being frequently used, Borneo was often touched at during the remainder of the 16th century, and trade relations with Brunei were successfully established by the Portuguese.