The islands of maritime Southeast Asia can range from the very large (for instance, Borneo, Sumatra, Java, Luzon) to tiny pinpoints on the map (Indonesia is said to comprise 17,000 islands). Because the interior of these islands were jungle clad and frequently dissected by highlands, land travel was never easy. Southeast Asians found it easier to move by boat between different areas, and it is often said that the land divides and the sea unites. The oceans that connected coasts and neighboring islands created smaller zones where people shared similar languages and were exposed to the same religious and cultural influences. The modern borders created by colonial powers—for instance, between Malaysia and Indonesia—do not reflect logical cultural divisions.
A second feature of maritime Southeast Asia is the seas themselves. Apart from a few deep underwater trenches, the oceans are shallow, which means they are rather warm and not very saline. This is an ideal environment for fish, coral, seaweeds, and other products. Though the seas in some areas are rough, the region as a whole, except for the Philippines, is generally free of hurricanes and typhoons. However, there are many active volcanoes and the island world is very vulnerable to earthquake activity.