Hydraulic fracturing is commonly known as 'fracking' and refers to a process used to extract natural gas from the ground.
It involves pumping a mixture of water, chemicals and sand into the ground at high pressure to release gas from shale rock formations.
The method is widely used in the US where it has produced cheap natural gas, but it has also met with resistance from local communities and environmentalists.
More top news
The Hollywood actor and environmental campaigner met the president-elect to discuss the economic advantage of green policies.
The Pirates of the Caribbean star brought in just $2.80 (£2.21) at the box office for every dollar he was paid for his last three films.
Junk food advertising is to be banned across all children's media - including online and social - in a bid to tackle childhood obesity.