The city also became an important military center of the Soviet Union. More than a dozen military schools and academies were established here, also specializing in high-tech warfare (see also Soviet education). This created a labor force demand which fed migration from rural areas of both Ukraine and Russia. Large suburbs and an extensive transportation infrastructure were built to accommodate the growing population. However, many rural-type buildings and groves have survived on the city's hills, creating Kiev's image as one of the world's greenest cities.