A year ago this month, an explosion in the Gulf of Mexico's Deepwater Horizon oil platform cost the lives of 11 men. It also caused the largest marine oil spill in history and wrought havoc on the southern coastline of the United States.

The political fallout was considerable and the battle over responsibility and compensation will drag on for years. But the disaster also gave rise to much debate about the human and environmental costs of oil production – particularly in offshore waters.

The argument has become particularly heated in the country from which the Gulf draws its name. Mexico is a major oil producer and the huge profits from Pemex, its state-owned oil giant, have long underpinned the country's economy.

Now, with some of its most productive inshore fields beginning to run dry, Pemex is pushing exploration out into ever deeper waters. And that, say many activists, is worrying because the company's safety and environmental record is not as good as it should be.

Filmmaker David Browne has been finding out what has got them so concerned.

This episode of People & Power can be seen from Wednesday, April 13, at the following times GMT: Wednesday: 2230; Thursday: 0930; Friday: 0330; Saturday: 1630; Sunday: 2230; Monday: 0930.