France, June 5 -- It's late morning on a windy day in early May on the quayside in Nantes, western France. Cloudy skies promise rain. But an 18-metre high wooden mast, overlooking the Loire river, stands out like a beacon.

Known as the Mast of Fraternity and Memory, it's a memorial to the estimated 550,00 enslaved Africans who were shipped from Nantes to the Americas and the Caribbean as part of the transatlantic slave trade between the 17th and 19th centuries.

Dieudonne Boutrin, a 61-year-old activist whose ancestors were enslaved on the overseas French territory of Martinique, and Pierre Guillon de Prince, a descendant of Nantes slave traders, call it their "baby".

"We chose a ship's mast because without ships, no one would have been...