Which geographic feature exists in less than 20 places worldwide as of 2017?

Mountains over 20,000 feet in elevation are indeed a rare geographic feature, with only a limited number of them found worldwide. As of 2017, there are fewer than 20 peaks that reach or exceed this height, primarily located within the Himalayas and the Andes. This elevation represents a specific climatic and geological condition that is not common globally, making these towering mountains stand out in the context of physical geography.

Persistent lava lakes, while fascinating, are more widespread. They can be found in several volcanic regions around the world, thus exceeding the threshold of 20 locations. Similarly, places where three countries' borders meet occur in various locations worldwide, as there are multiple instances of such tripoint formations due to the complex nature of international borders. River deltas are common geographical features formed at river mouths and can be found in many regions, indicating that they also do not fit the criterion of being limited to fewer than 20 examples globally.

In summary, the rarity of mountains that rise above 20,000 feet uniquely qualifies them as a geographic feature that exists in less than 20 places worldwide.