OK, pop quiz. Name the best ten documentary films you saw at your local cinema last year. How about ten documentaries that played at your local cinema last year? That were released for cinema showing? Documentary is too often seen as a televisual format, but many of the finest documentary works were made for cinema distribution. Frederick Wiseman made a career as a film documentarist with works such as Meat, Hospital and the groundbreaking Titicut Follies. English director Nick Broomfield has developed a distinctive style of his own, seen to fine effect in such works as Chicken Ranch, Biggie and Tupac and Aileen Wuornos: The Selling of a Serial Killer. And let's not forget Errol Morris, whose Thin Blue Line remains one of the most compelling examples of the genre, and Michael Moore, who has achieved widespread acclaim with Roger and Me and whose Bowling for Columbine packed out cinemas across the world.

The best documentaries take us places that we have never been, show us a side of life we did not know existed or directly challenge the established status quo and those supporting it, presenting an alternative viewpoint of sometimes unjustly accepted attitudes or beliefs. It is often the desire to expose the truth that drives documentary film-makers, and their credibility as film artists, as they use the medium to campaign or communicate rather than to profit or further their own careers, is subsequently often stronger than their counterparts in film fiction. This was certainly the case with young British-born Marc Singer. Having spent some time in New York, he heard about a group of homeless people who had built a small community in the Amtrack rail tunnels beneath the city, complete with makeshift housing and jerry-rigged electricity. After getting to know them and actually living with them a while, he decided to make a film about them and their lives. The result is the remarkable Dark Days.

The situation itself is so extraordinary, and for most viewers so surprising, that Singer is able to spend the twenty minutes or so that make up the first act just introducing us to the varied characters and the makeshift underground village they have created. All are remarkable for their resourcefulness, comradeship and good humour, finding opportunity in the most unlikely situations and even looking on homelessness as a form of freedom. We are a third of the way through the film before the darker side of their story begins to emerge – the drug addiction, the rats, the lack of sanitation, the characters' sometimes heart-rending back stories. The third act is unexpectedly narrative-driven, with the intervention of city authorities bringing about a momentous change for all living in the tunnel, one that will have a dramatic effect on all of their their futures.

Make no mistake, fascinating though the subject matter is, the film does not trade on this alone. Having lived in the community and won the trust of its naturally cautious inhabitants before the idea of making the film was even suggested, Singer gets closer to them than any professional crew could have. As a result, all of those in the film react to Singer not as subjects under his observation, but as friends. Despite the title, this is anything but a dark movie – it is an enlightening, uplifting and inspiring one, a compelling, genuinely moving testament to the strength of the human spirit.

Shot in 16mm on what looks like Tri-X, a high contrast black-and-white stock, this was never going to be reference material, but even in the cinema this seemed wholly appropriate – a glossy, well-lit, grain-free colour picture just would not have worked anything like as well. This is recreated most effectively on the DVD – indeed, considering the source material the transfer here is impressive. Framed at the theatrical projection ratio of 1.85:1 and anamorphically enhanced, it takes director Singer, on the commentary track, to remind us that the original aspect ratio was 4:3 and that it has been cropped for theatrical distribution. He expresses a preference for the original framing, but there are no examples to compare and the 1.85:1 framing looks fine and never seems too tightly cropped.

Two mixes are included, Dolby 2.0 stereo and Dolby 5.1 surround. The 2.0 mix is centrally weighted and on paper the closest to the film's low budget identity, but the 5.1 mix is seriously superior and does actually add to the film. Though the dialogue is all confined to the centre speaker, the ambient sound is much more expansive, spread across the front stage (there is no real rear speaker action) and helps to create a sense of place, pulling us into the tunnel far more effectively than the 2.0 mix. Bass on the 5.1 mix is also very effective in certain scenes, such as when Dee's house burns down or during the opening shots of trains. D.J. Shadow's evocative, low-key score definitely belongs in 5.1.

Another example of a low budget independent film that has been given four-star treatment. In such cases it is often the involvement of the director himself in the disk that ensures a quality release, and this is clearly the case here.

So often the inclusion of a 'making of' documentary will prompt a groan from DVD enthusiasts, as it is almost always little more than an electronic press kit, a ten minute collection of sound bites from principal cast and crew members intercut with a few shots of them working on the film and some extracts from the finished work. Actors tell us what sort of characters they are playing, directors talk about what they "wanted to do with this film" and everyone says what a great project it has been to work on. Insight value: zero. Fortunately, this is not always the case, and just occasionally you get a look at the making of the feature that is revealing, compelling and a decent length. The Making of Dark Days – A True Independent most definitely qualifies. 46 minutes long and shot on 4:3 video with a Dolby 2.0 soundtrack, the story that unfolds in this straightforwardly filmed but compelling extra is almost as remarkable as that in the main feature. Director Singer and others from various stages of the production reveal how someone who had never held a camera in his life and had no film training whatsoever brought an award-winning film to the screen through a series of sometimes amazing chances and favours. Almost everything, from the original conception and obtaining equipment to lighting the tunnel, creating a dolly and even getting the film processed, has a fascinating tale attached to it. An excellent extra that compliments the film perfectly.

The feature commentary by director Marc Singer inevitably repeats some of the stories found in the documentary, but still delivers a great deal of new information about the characters and the process of making the film. There are very few dead spots and Singer has an engaging delivery style, never for a second coming over as pretentious or self-important. Singer clearly cared a great deal for those in his film and this comes across in spades.

A total of fifteen Deleted Scenes are included, most of them quite short, all of them featuring the main characters from the film. Though no startling new information is provided by these scenes (otherwise, we presume, they would have been included), all the extra material is interesting, some of it very funny, and it helps us connects even more effectively with the characters, which itself helps serve the purpose of the film. Every deleted scene is proceeded by context-setting introductory notes from Marc Singer.

A History of the Tunnel by Margaret Morton is a textual history of how communities developed within the tunnel. Though short, it is still an essential read.

Life After the Tunnel is a text-based feature in which Marc Singer relates what has happened to all of the principal characters in the time between the completion of the film and the release of the DVD. Unlike many text features, this is essential reading and provides a sometimes inspiring, sometimes sad postscript to the film. Definitely do not read this before seeing the main feature.

Short Biographies of the film-makers are provided, as is a very simple, 1 minute, 4:3 trailer for the film and three web links to relevant internet sites.

Selecting the Palm Pictures logo on the main menu will give you the only out-of-character extra on the disk, a 4:3 trailer for one of their other releases, the anime feature Blood: The Last Vampire.

Dark Days is essential cinema, a compelling, eye-opening documentary and a genuinely humanist work that takes us to the underside of society and blows away our preconceptions, forcing us to re-evaluate our view of the homeless, which was director Singer's intention in the first place. The DVD is very well specified, sporting a strong transfer, a fine soundtrack, a splendid making-of documentary and a fascinating commentary track. Highly recommended.