This immense Continent was Submerged after the Cataclysm that happened ~11,600 years ago ending the Last Ice Age. A very different geographical world was revealed because the Sea Levels were roughly 100 meters lower than today.   Sundaland was as big as the whole of the United States, including Hawaii and Alaska.  

Here is the Landmass that was believed to be exposed during the Last Glaciation Period.

Here the equivalent of the Mercator Map on how the World looked during the Last Ice Age.   North America and most of Europe and top Asia were Covered in Ice.

Here with official images released of the ocean´s seafloor, it becomes obvious how Indonesia would become an immense continent during the Ice Age.

Map of the many volcanos, Indonesia sure is the Land of Fire.