The Tunnel is a horror film produced in Grand Theft Auto III. It's based on the Porter Tunnel as evident from the movie posters. The poster also features a beta Sentinel in the tunnel with the tagline of the movie reading;
|“||It's Dark Down There...In The Tunnel||”|
Holding the tagline of the movie, the genre is assumed to be horror. The tagline could also hint towards an inexplicable or dark force wandering in the tunnel but no theory has come across to be authentic due to the lack of references in the game itself with the cryptic tagline, Porter Tunnel, and a beta Sentinel.
It has been theorized that the tagline contains a cryptic context about mole people. Homeless people are part of an urban legend in real life Liberty City, these people are rumored to be the homeless living under tunnels in groups. Over the time, they have been linked to ghosts and vague monsters. However, the theory isn't authentic because of no in-game or technical confirmation.
Ironically, the movie was hinted again in Grand Theft Auto IV by a security guard at the end of the Booth Tunnel in Alderney. The security guard speaks vaguely that he didn't want to pursue the job due to fear as the movie "The Tunnel" was filmed in the very tunnel. This particular piece of hint and continuation of the mystery may help to garner more collective evidence but the mystery remains unsolved for now.