Otjikoto Lake
Otjikoto Lake is a the smaller of only two natural lakes in Namibia. It is a sinkhole lake, created by a collapsing karst cave, located 20 km outside of Tsumeb, a few meters from the main road B1. The lake was a dumping ground for German troops during World War I; German troops dumped war materials in the lake to stop the South African and British troops from using them. Most of the materials have been recovered and are displayed in Tsumeb Museum. Tilapia guinasana, a mouth-breeding species of fish which naturally was only found in Otjikoto's sister lake, Lake Guinas, was introduced to Otjikoto Lake. The claim that lake Guinas is indeed connected to lake Otjikoto by underground caves is frequently made but not proven as yet. Source: Wikipedia.org