Seneca wrote a book, De Nubibus in "Naturales Quaestiones", that gave details about a Nero expedition to the «caput mundi investigandum» (to explore the top of the world) in 61/62 AD. In this book he explained to have reported what two legionaries told him about their discovery of the «caput Nili» (the origin of the Nile river): «Ibi Vidimus duas Petras, ex quibus ingens vis fluminis excidebat…ex magno terrarum lacu ascendere…» (We saw two huge rocks, from which the power of the (Nile) river went out in a powerful way.....(The Nile river) comes from a very huge lake of the (african) lands). Furthermore Seneca wrote that the legionaries told him that the water of the Nile river, that jumped through two huge rocks, was coming from a very big lake inside the African lands.