Western sanctions on some oil producers, like Iran, Russia and Venezuela, have caused a growing number of oil tankers to “go dark.” That is, to shut off the tracking system that could alert authorities of their movements.

A new study from the National Bureau of Economic Research estimates a whopping 43% of seaborne crude oil exports traveled on “dark ships” in recent years.

That 43% is just an estimate because it’s hard to get an exact number. But the fact is, a whole lot of sanctioned oil is still moving around the global economy.

If you’re piloting an oil tanker and you shut off your transponder, you don’t disappear altogether. 

“This is not Star Trek. We don’t have cloaking devices,” said Ellen Wald of the Atlantic Council. 

But Wald said it does make it harder to find you, which might involve combing through satellite photos and the like.

“It’s just tedious,” said Wald. 

So ships go dark to try and hide their cargo. This often happens when two oil tankers rendezvous at sea, said Ian Ralby, CEO of the maritime consultancy I.R. Consilium.

“ If you have a small tanker, that takes sanctioned oil out from Iran, and transships onto a much larger tanker that is carrying a cargo of legitimate oil, they can obscure the fact that that oil is partially sanctioned,” said Ralby. 

And it turns out it’s not hard to find a buyer for oil of questionable origin. Erica Downs, an energy researcher at Columbia University, said a lot of dark-shipped oil ends up at small, independent refineries in China.

“ These refineries operate on very thin margins and they’re highly opportunistic crude buyers,” said Downs. 

This means they’ll take the lowest cost crude, sanctioned or not. Oil on dark ships also winds up in South Korea, India and Egypt, according to the National Bureau of Economic Research study.

This all means that sanctions haven’t really squeezed the oil market, said Robin Brooks of the Brookings Institution.

“Global oil supply was not at all materially impacted,” said Brooks. 

He said neither was the price of oil. What has happened, with so many major oil suppliers under sanction, is that the global oil trade has basically split into two parallel channels, said Ian Ralby of I.R. Consilium.

“ If you put everyone outside of the tent, they’re just going to make their own tent. And that’s essentially what we’ve done. We’ve created a new marketplace for sanctioned actors and their enablers,” said Ralby. 

Another unintended side effect, said Ralby: these tracking systems are a safety feature to help ships avoid collisions. It’s not a great idea to turn them off.