Non-defining relative clauses practice	

The Tunguska Event	It occurred on June 30, 1908.	It was a powerful explosion in Siberia.
It is believed to have been caused by a meteorite.	
The London Underground	It is popularly known as 'the Tube'.	It serves 270 stations and comprises over 400km of track.	
The Suez Canal	It was opened in 1869.	It is 192 kilometers long. 
It links the Mediterranean with the Red Sea.	
The Taj Mahal	It was completed around 1653.	It was built as a mausoleum for his favourite wife by the Mughal Emperor Shah Jahan.	
The Vatican City	It has a population of around 900.	It is the smallest state in the world.	
The Berlin Wall	It was started in 1961.	It was a symbol of the Cold War.	

Answers	

The Tunguska Event, which occurred on June 30, 1908, was a powerful explosion in 
Siberia (that is) believed to have been caused by a meteorite.

The London Underground, which is popularly known as 'the Tube', serves 270 stations and comprises over 400km of track.

The Suez Canal, which was opened in 1869, is 192 kilometers long and links the Mediterranean with the Red Sea.

The Taj Mahal, which was completed around 1653, was built as a mausoleum for his favourite wife by the Mughal Emperor Shah Jahan.

The Vatican City, which has a population of around 900, is the smallest state in the world.

The Berlin Wall, which was started in 1961, was a symbol of the Cold War.