Martin Compston's new Scotland based thriller has already proved a hit with fans after it aired for the first time on Sunday evening in BBC One's prime time spot. 

Episode two airs on Monday evening, and thousands of people will be tuning in after the series opener left them with some unanswered questions. 

Filmed in Glasgow and around the west coast of Scotland, much of the action revolves around Dunloch naval base and the HMS Vigil submarine. 

But is Dunloch a real place in Scotland? Was the action really filmed on a submarine?

Here's everything you need to know...

Is Dunloch naval base as real place?

Dunloch naval base featured heavily in last night's episode of Vigil, leading people to question whether Dunloch was in fact a real Scottish town. 

Unfortunately, fans will not be able to visit Dunloch as it is a fictional town created for the show. 

However, while Dunloch might not be a real place, it is based on Faslane, the HM Naval Base Clyde on the Gare Loch in the west of Scotland which houses the UK’s nuclear deterrent Trident and its four-strong fleet of Vanguard-class submarines.

Was Vigil filmed on a real submarine?

With a huge proportion of the show set on the HMS Vigil, you may be wondering if filming took place on a real submarine...

The answer is it did not - instead, producers and set designers worked very hard to recreate the atmosphere of a submarine. 

Speaking on the difficulties surrounding setting, director James Strong said filming on a submarine was "largely impossible", describing the recreation of such surroundings as "an enormous endeavour". 

"First of all a there was a Trident submarine - the setting for the majority of the show. I like to shoot on real locations - but for obvious reasons this was largely impossible, so we had to create a big enough space to contain the action and be big enough to work in and yet retain all the claustrophobia of a real submarine.

"Creating HMS Vigil was an enormous endeavour, brilliantly carried out by designer Tom Sayer and his team. It looked incredible, was a great place to shoot and yet always felt utterly real."

What's a coxswain?

You might have also heard some discussion about a coxswain during Vigil...

The coxswain is the person responsible for steering the boat or ship, as played by Shaun Evans in Vigil. 

In the Navy, the coxswain is a senior petty officer in a small ship or submarine.