The waste was generated as the result of an oil deal spanning three continents. Trafigura bought a consignment of cheap and dirty heavy oil with a high sulphur content. Instead of putting it through a refinery, Trafigura tried to clean it up, using a do-it-yourself method, so they could sell it on at a massive profit.

They used a ship called the Probo Koala which they stationed off Gibraltar as a rough and ready refinery. Caustic soda and a catalyst were added to the oil which reacted with the sulphur and settled to the bottom of the tank. Trafigura were then able to sell the oil, but left with a toxic sludge at the bottom of the tank.

The Probo Koala went to Amsterdam where they attempted to unload this sulphurous tar as if it were normal ships' waste, which would have cost a few thousand Euros.
... BBC