Why is so much oil being stolen in Nigeria?

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Nigeria's state-owned oil company says an oil tanker capable of carrying 800,000 litres of smuggled crude has been intercepted offshore while heading to Cameroon. Nigeria's state-owned oil firm said the oil had been stolen from a well in the south-western state of Ondo. As dramatic as this finding was, it is just a fraction of the oil being stolen in Nigeria. So, what is behind it?