Which regions was still considered to be frontier?

As of the early 21st century, regions still considered to be frontier areas include parts of Sub-Saharan Africa, the Arctic, and certain remote areas in South America and Southeast Asia. These regions often have limited infrastructure, lower population densities, and untapped Natural Resources, making them less developed compared to urban centers. Additionally, some areas in the western United States, such as rural Montana and Wyoming, retain a frontier character due to their vast open spaces and low population.