internal wave

 Waves that we see on the surface of the ocean are created by the friction created by a current of air connecting with the water. This imparts energy to the water causing it to rise up in a circular motion, coming back to the original level when the circle is completed.

 But the internal wave is more akin to the pattern created by a ball rolling off a step and then bouncing back up, falling and bouncing up repeatedly.  This is what happens when a current of water below the surface falls off a hill on the ocean floor. The current of water then bounces back up, falling and bouncing repeatedly. This creates quite a lot of turbulence in the surrounding water.

 The waterways around Indonesia are susceptible to internal waves because there are a number of narrow channels from which strong currents of water fall into deep troughs.  The currents are so strong that they create a very particular kind of internal wave known as the internal standing wave.  A standing wave is one which is stationary relative to the ocean or river floor, so it just goes up and down in the same spot.  It can occur when two currents meet.  The internal standing wave is a standing wave that is happening underwater.  The internal solitary wave is a single hump-shaped wave that can travel long distances before it rolls over, rather like a wave on the beach, at which point it breaks up and dissipates. It can be hundreds of metres high and kilometres long, and occur at a depth at which submarines operate. The Indonesian submarine which sank to the ocean floor had encountered one of these internal solitary waves and was carried downwards by a force too great for it to resist to a depth below its maximum operational depth.