Imbedded in the sand dunes are the Mandara-Lakes like jewels in the area of Um al Maa (near Germa). Depending on the groundwater table several smaller lakes are formed and create a landscape of oases. This view is a feast for the eyes and actually appears as a mirage. The lakes are partly fed from hot springs. But this will not do harm to the swimming, if you are not too sensitive regarding midges and the salt content.
This destination is absolutely to recommend for every tour towards this area. Instead of a lot of words, here rather some pictures