The equator lies about 100 miles south of Kuching, and most of northern Borneo receives over 3000 mm of rain each year. Borneo is naturally covered by tropical rainforest. This covers the mountainous areas cut by many rivers with very steep-sided hills and hilltop ridges often only a few metres wide. The high rainfall means large rivers; these provide a principal means of transport and are formidable tactical obstacles. Dense mangrove forest covering vast tidal flats intersected with numerous creeks is a feature of many coastal areas, including Brunei and either end of the border. There are cultivated areas in valleys and around villages. In the vicinity of abandoned and current settlements are areas of dense secondary regrowth.