Vying with another small spring in Nyungwe Forest National Park in Rwanda for title of the source of the Nile (the truth of the matter is that there's no one source), this insignificant-looking little spring at Kasumo, 115km southeast of Bujumbura, might be the southernmost source du Nil. In a nice touch, a stone pyramid marks the site, but unless you have your own transport it is almost impossible to reach.
If you fancy a swim there are some hot springs a further 10km south.