For those of us crusty enough to have been alive in the UK in the early 1990s, the best way of grasping what was going on in heavy guitar music was to stay up until 3 AM on a Thursday and watch the ITV metal hour, Raw Power (times may vary subject to regional variations). Here, Def Leppard, G’N’R and Enuff Z’Nuff would be rammed together with Soundgarden, Mudhoney and the Smashing Pumpkins, as well as a smattering of misguided bands who tried to bridge the hair–grunge gap, like Sven Gali and Warrant circa Machine Gun. But most interesting was the stuff on the fringes that didn’t really belong anywhere (T-Ride, anyone?). Most notably, a disturbing Jan Švankmajer-style stop-motion video in which a small, dusty man in a Kafka-esque subterranean space underwent a grubby existential nightmare involving a pipe crammed full of glistening flesh. Finally, he crashed to the floor, his nigh-featureless face melting away on impact. Unlike the old, peacock-haired bands and the new, lank-locked ones, this mysterious lot were (like the dusty wee man) faceless, their music unsettling, brittle and alienated in a way that the grunge bands idealised but never quite realised.

This description of Tool may not be one that many people recognise. Ever since Aenima (1996), they have been wildly and unashamedly prog in all respects and on a colossal scale – huge songs, massive concepts, weighty symbolism. Yet 1993’s Undertow is in many ways the work of a very different band. You’d be hard-pressed to find members of the Tool Army who’d describe it as their best work. But conversely, it’s relatively free of the trippy portentousness reviled by their detractors.

Unlike their grandiose later albums, Undertow is almost painfully dry and austere. A key reason for this is the foregrounding of the bass, but not in the wanksomely ostentatious way of Primus or the Chili Peppers. Original bassist Paul d’Amour is the foundation stone of the overall aesthetic – his is a clanking, metallic tone, awash with harmonics, that resembles a gremlin hammering a radiator rather than a stringed instrument. His contribution nudges the band’s aesthetic towards old-school Test Dept industrial or the ferrous noise rock of Big Black. Latter-day Tool would be bigger, certainly; heavier, no doubt; cleverer, indubitably – but not this vicious, this razor-edged and wiry.

The other key element, then and now, is Maynard James Keenan’s voice – for which, at the time, there was no real precedent, in rock music or beyond. Love it or hate it, it’s a remarkably distinctive and powerful throat, capable of simultaneously conveying primal force and literary erudition, naked vulnerability and steely rage in just one note, one syllable.

Emerging from 25 seconds of near silence and desperate gasping, the intro riff to first track ‘Intolerance’ seems to start on an a disorienting off-beat, as if you’ve come in partway through. It’s a confrontational and confident opener, a statement that the band have no need to prove themselves, no need to establish their identity, because their being is extrinsic – it predates the moment you chose to press play. After this point, the song never quite settles on either riff or rhythm, its appeal lying in the way it lurches gracefully between jagged, percussive passages and cathartic detonations. ‘Sober’, the breakthrough song, is understated, sombre and minimal, an extremely curious choice for a first single and an unlikely MTV hit. Much of its success – and consequently, Tool’s elevation to the prog/metal pantheon – must be attributed to the iconic video mentioned earlier, by guitarist Adam Jones, a visual artist and animator of considerable skill and wider renown. Lyrically, however, it perfectly reflects/parodies/capitalises upon the self-obsessed moping of the grunge era: ‘I am just a worthless liar; I am just an imbecile’.

The turbulent riff of 'Bottom' also reflects its era, the most grunge-obvious and instantly appealing motif here. It also features one of the album’s most memorable episodes, when a cartoonishly ascetic incarnation of Henry Rollins turns up to grimace manfully: ‘I’ve gone to great lengths to expand my threshold of pain. And I will use my mistakes against you'. In its duration and transitions, it foreshadows the long-form style that would come to epitomise Tool in their maturity – only harder, leaner, less self-consciously progressive and virtuosic, revelling in sheer bludgeon. At the apex, it momentarily switches into higher thrashing gear, a brief interlude echoed elsewhere on Undertow. Each instance is a crack in the mid-paced monolothia, hinting at seething energy below the tightly controlled surface, echoing the tensions embodied in Maynard’s voice. There’s a conflict here between the animal and the intellectual, arguably the last time this push-and-pull would be found in Tool’s work. From Aenima onwards, the intellectual would win out, the animal would be caged.

Prior to track five, you could dismiss Tool as slightly eccentric grunge also-rans, but from here onwards they spiral out into weirder territories. ‘Crawl Away’ and ‘Swamp Song’ both ride awkward, stuttering, trigonometrical riffs and consist of a succession of rolling, slippery rhythms, liquid transitions and dynamic outbursts. In their use of odd timings and sprawling, non-traditional structures, these tracks foreshadow Tool’s future big prog adventures, but their vocabulary is far leaner and more severe. Oddly, ‘Crawl Away’ includes a suspiciously cheesy out-of-place traditional heavy-rock scream from Maynard: ‘Get up, get up, get up, NOOOOOOW!’ – an inexplicable nod to Ted Nugent’s ‘Stranglehold’, which is pretty much the least acceptable reference a musician could make in 1993. Or, indeed, now.

Heard in isolation, the title track’s main and coda riffs are both unremarkable for their time, in that either could have been lifted from Badmotorfinger, but the transition between them is one of the album’s defining moments. Maynard vomits up an inhuman roar as the song crashes uncomfortably to a trudge, and holds a queasy quarter-tone at the grinding climax. The first four-and-a-half of ‘Flood’ take the album’s heaviest, doomiest, most transcendent riff, stretch it, break it and repeat it. This is the closest Tool would ever get to the sound of their old friends Melvins, echoing, intentionally or otherwise, the tar-drinking opener to the previous year’s Lysol. When a circular riff breaks free and the song ‘proper’ starts, it’s both a relief and a disappointment.

For all their intensity and air of lofty seriousness, Tool have always had a paradoxically impish joy in puzzles, a clunky sense of humour and a fondness for spinning bollocks. During the Undertow period, they claimed that their primary influence was The Joyful Guide to Lachrymology, written in the late 1940s by otherwise crop-sprayer Ronald P. Vincent. This succinct publication, it was alleged, espoused the theory of achieving enlightenment through pain and tears. Even now, it’s uncertain whether Tool just made up this book – it was eventually reprinted, but that does little to waft away the stench of hoax. The lachrymology angle was rarely discussed after the early days, which gives the whole thing the feel of an ill-conceived gimmick.

As overwhelmingly bleak as Undertow is, this arsing-about element dominates the album’s last quarter – after a prolonged silence, we arrive at track 69 (which is a bit rude, ho ho), ‘Disgustipated’. Maynard, in cartoon preacher mode, delivers an uncomfortably glib spoken word piece in which harvest day is seen from the carrots’ perspective as tantamount to the Holocaust. The subsequent psychedelic/industrial anti-hippy carnivorous drum circle (‘This is necessary/Life feeds on life feeds on life feeds on life’), complete with the sound of shotguns and pianos being destroyed by sledgehammers, is one of the most inventive and distinctive tracks, but the preceding monologue renders it both trivial and worrying. We then get seven minutes of cicadas chirping and another spoken-word piece – a rambling story left on Maynard’s answerphone. As would happen more than once over the next decade or more, Tool’s finer instincts eventually give way to obfuscation, absurdity, padding and lengthy buggering about.

Following on from the intriguing but slightly underwhelming Opiate EP, Undertow found Tool arriving seemingly fully formed, even though the vast leaps they made on subsequent releases would dispel this notion. Crucially, Undertow contains within itself not only the seeds of their future selves, but – in its severity, feral character and bony-elbowed awkwardness – the template for an entirely different band that never quite came to be.