A large part of the human history remains unknown because the Sahara is very difficult to explore and study.
Who knows, maybe entire city states are hidden here and there within endless quantities of sand.
Indeed, one great path to finding those lost civilizations is to follow river vallies, Atlas pro[0] says there is no consensus on past rivers/bodies of water as evident by different maps of the region, but one can follow (underwater) river sediment which were deposited to the sea. During oil search such area was found in Cap Timiris, Mauritiania. Satelite radar image of the area from 2015 did reveal those rivers beds, Tamanrasset River seems to be the biggest, with a big drainage basin. Trees and shrubs indeed grow there.
I would think satellite observations would have thoroughly mapped the immediate subsurface by now regardless of it being hard to explore conventionally.
Subsurface? No. Sahara is too vast and research is expensive. They can barely get funding to do that sort of work where they already know there is a Mayan temple under their feet.
