Sludge Metal has the merit of being a musical genre that sounds exactly as the name implies.
Sludge metal is dense, thick, rolling, and relentless. Listening to it feels like bubbling hot sewage is being forced into your ears. The instrumentation is characterized by down-tuned, distorted guitars, grooving deep bass, and barked hardcore vocals. Sludge is mixed fairly evenly, with vocals slightly lower in the mix and drums and bass higher than in convention. The combination of the down-tuned guitars, relatively high bass, and deep vocals means that the whole thing hits you on the low-mid frequencies. The length of the groove, and the chugging of the engine-like guitars means it grabs you right in the chest and turns your stomach, as if you yourself, were turning into the slowly rotating arms of a waste treatment plant.
Song structure can vary from band to band, and again across the vast number of sub and related genres. The band SUNN O))) have been known to drop to tempos as low as 1 BPM over their 20 minute long drone-doom epics. At this stage chord progressions take minutes instead of seconds, notes barely exist under the distortion and static and everything gets rather silly. More accessible bands such as Mastodon and Baroness are happy to write three minute chorus-verse-chorus-solo-chorus-chorus songs, with the pace of the average thrash record. This is often what leads people into the genre.
Typically you can expect songs to be slower, and longer, with non-conventional song structures that evolve and develop. You can expect instrumental or ambient breaks as well as tempo changes, electronics and atmospherics. The main feature of any sludge track will be, of course, the crushing metal riffing that twists, turns, rolls, rotates until it eventually mutates into a song. Ultimately, and comfortingly, the only thing that truly unites music under the Sludge banner is if sounds like it.
If Black Metal is one face of the Extreme Metal coil then Sludge is the other. Rather than a focus on burning churches and dressing like clowns in a forests, Sludge's roots are in hardcore punk and doom. More recently it has seen influences from post-rock, which seem to have completed the genre and given it a focal point and contrast needed to bring it above pure anger and teenage angst. Lyrical content is often focused around depression, abuse, decay and politics. This is a thousand miles from the miscellaneous wizards and swords of Dragonforce - and such it is an entertaining experience to see a Sludge band take stage at a metal festival.
If you're not a metal fan, this is probably all sounding pretty terrible. In some ways it is. It has the same ridiculous self-fulfilling theatrics of all other metal. But don't be fooled into dismissing it right away. The contrast between the expansive, open instrumental sections and the crushing, claustrophobia of the loud sections is almost therapeutic to experience. You have to surrender yourself to the sound, and simply try to ride the waves of black noisy treacle to the shore. Down-tuning guitars and massive distortion isn't just about turning it up to 11. With repeated listens you'll find the atmospherics and subtle touches. The rattle of a tambourine isn't something someone that is just making noise would bother with. Many Sludge artists, though dealing with dark themes, are talented and intelligent, with valid artistic points to make. They are also not afraid to show it. The band Cult of Luna, last release was an audio book, and though not much to do with Sludge, it was still beautiful, artistic and exceptionally good.
Go to any modern art museum and you're likely to find an exhibition by an artist, with some intent to offend or disgust. About a year ago I went to an art gallery and sat in a room, watching a video of a woman in a bondage suite, tied to the ceiling of a dirty hallway, attempting to cover herself in vaseline. I can't fully relate to, or understand this form of art...but it is popular, and I can't help but think the natural musical counterpart in extreme metal, has somehow been dismissed as kids dressing up and men in mid-life crisis'.
The future of Sludge is unclear. Though people like the Melvins have been putting out material since the 80s, it is still a refreshingly new genre. This is particularly true on the post-metal side of things, which really only originated in 2002, with the release of ISIS' landmark album Oceanic. Unique and innovate material, such as the aforementioned audio book, is still being pushed through the ranks. With the Coldplays and Justin Biebers of this world, and the general attitude toward metal, I don't think commercial success or artistic recognition is going to be a serious option any time soon. Even so, if you wish to be hip and on the bleeding edge of metal, I suppose this is where you should be.
So, if Sludge still sounds interesting after all that, perhaps you should check out Salvation by Cult of Luna.
Or if the vocals really are a turn off, you could take a look at the instrumental and softer Australasia by Pelican.