Periods of heavy rainfall filled the crater with water, causing the lake level to rise above the lowest points of the rim. Such periods are evidenced from fossils of fish found on hilltops. Water even flowed from the basin through an overflow channel. However, there were also times when the water level was so low that the rainforest entered the basin rendering the lake only a small pond. Such a period, according to legend and now proved by paleoclimate records, lasted until about 300 years ago (Shanahan et al. 2009). Lake Bosumtwi can be used to argue that the drought over sub-Saharan Africa and the Sahel during the late 1970s originated from that of natural variability forced by a multi decadal oscillation in the thermohaline circulation, which in turn modulates sea surface temperature in the north Atlantic. This evidence shows the drought occurred via natural origins instead of anthropogenic forcing. It has been shown that historical droughts in this region were four times drier than that of the late 1970s drought
