Mexicans make several cultural subdivisions within the nation. The most common one identifies northern, central, and south or south-eastern Mexico. The extensive and desertlike north was only sparsely populated until the middle of the twentieth century, except for some important cities such as Monterrey. It has traditionally housed only small indigenous populations and is generally regarded as a frontier culture. Densely populated central and western Mexico is the cradle of the nation. Highly developed Indian cultures populated this region in pre-Columbian times and it was also the heart of the colony of New Spain. Many prominent colonial cities are major urban and industrial centers today. Southern Mexico has a tropical or subtropical climate and some rain forest. It is characterized by a strong indigenous heritage and is also the poorest part of the country.