The topography of Croatia is geographically diverse, with flat plains along the Hungarian border, as well as low mountains and highlands near the Adriatic coast. The country is generally divided into three main geographic zones: the Pannonian and Peri-Pannonian Plains in the east and northwest, the central hills and mountains, and the Adriatic coast. Approximately 24% of Croatia's land is arable. Croatia's natural resources include: oil, some coal, bauxite, low-grade iron ore, calcium, natural asphalt, silica, mica, clays, and salt. Croatia's natural environment experiences effects from frequent earthquakes, air pollution from metallurgical plants, coastal pollution from industrial and domestic waste, and forest damage.