Who blew up the Nord Stream pipelines?

The controversial Nordstream pipeline that connects Russia to Germany made headlines last September when segments of it mysteriously exploded, deep under water.

Who was responsible?

"My guess is the Russians," says German diplomat Christoph Heusgen tells Ian Bremmer on GZERO World.

But proving that suspicion, Heusgen acknowledges, will be much harder to do. "Objectively, it's going to be very, very difficult to do this research. It's at 100 meter below the surface...It will be very difficult to find out."

Watch the GZERO World episode: Europe’s tough decisions: Russia, China, and EU unity