Charcoal
Observation of IMDG Code on charcoal states "Liable to heat slowly and ignite spontaneously in air. The material as offered for shipment should have been sufficiently heat-treated and should be cooled down to ambient temperature before packing."
Some shipments offered as non-hazardous have also ended up buring at sea. Know your cargo, be compliant.
To know more about dangerous goods, visit https://shashikallada.com/