The words “slavery” and “oil”, when used together, are usually reserved for use in the context of palm oil, but the offshore oil and gas industry isn’t immune to modern-day slavery—as evidenced by the detention of a BP-charted offshore oil supply vessel in Scotland, which was found to have a group of unpaid workers on its crew.

The Malaviya Seven offshore supply vessel (OSV) was detained in Aberdeen, Scotland, earlier this week, with trade union representatives calling it a “blatant example of modern day slavery”.

Owned by Mumbai-based GOL Offshore, the vessel was chartered by BP from 1 June to 15 June. Among its crew were 15 Indian workers who had been unpaid for months.

According to the General Secretary of the National Union of Rail, Maritime and Transport Workers (RMT) the incident “exposes the shameful practices in the exploitation of our natural resources”.

These “ships of shame”, as RMT spokesman Mick Cash dubbed them, “are a blatant abuse of migrant workers […]” adding that such activities also act as “a catalyst for the dumping of UK seafarers, many thousands of whom are now drawing benefits from the state”.

The slave ship incident has now prompted another investigation into a second vessel owned by the same company, the Malaviya Twenty, which is at the British port of Great Yarmouth. Meanwhile, the Malaviya Seven will remain locked down at the Albert Quay until its staff are paid.

If you think slavery doesn’t exist anymore—think again. According to the International Labor Organization (ILO), almost 21 million people are victims of forced labor today. Of those, nearly 19 million are exploited by private individuals or enterprises, compared to slightly over 2 million who are exploited by the state or rebel groups.