It's worth bearing in mind the climate of Zanzibar. It's a tropical island six degrees below the equator. It has two rainy seasons with torrential rain and flooding. Annual rainfall is av. 1684 mm (compared to 1,168 in the UK). So Zanzibar gets nearly 50% more rain than the UK. So it can be very wet and humid. The average temp. is 25 - 30 degrees C.
So the sword would have been exposed to a lot of water while in the soil. Imagine what those conditions would do for between 300 and 500 years...
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