Near the ancient walls of Kano city in northern Nigeria is a huge lake, says the BBC's Andrew Walker.
The water is bright green and smells putrid. The lake is fed by a mixture of raw sewage and rainwater that filters through the rubbish, picking up chemicals and bacteria on the way.
On the rubbish tip that surrounds it children scavenge for things they can recycle like metal, bottles or jars.
Some duck to the bottom to get rubbish others cannot reach.