What are the positive aspects of mountains

In ancient times, mountains were used as a natural barrier (used for protection) from the weather and neighboring countries. They're neighboring countries may go into a "great depression" and seek for new natural resources. They may acomplish this by attacking near by villages, but if you were on the other side of the mountain from them, you were safe since it is very long and dangerous expedition.

Sonar is an application of the doppler effect used by submarines.  this test was on k12.