It happens at night, under cover of darkness. Thieves move into the mangrove swamps and creeks of Nigeria's southern Niger Delta region, homing in on the oil pipelines that criss-cross the area. They puncture the lines or open their valves, siphoning off crude and refined oil into barges or trucks. "Bunkering" - as this method of stealing oil is known - is taking a toll on local communities and the environment. After puncturing pipelines to steal oil, the thieves tend to leave them leaking. The resultant spills have proved detrimental for vast expanses of forests and farmlands in the Niger Delta - and have also led to fires.