It’s 11am on a Thursday and Millie Crampton is spending her lunchbreak explaining her double life to me. By day, she’s a student, studying mental health nursing. By night, however, she is Beryl, frontwoman of Basement Torture Killings, who deal in “serial-killing snuff grind”. Look on YouTube and you can find a video of her waving what appear to be some intestines around while singing The Rat Catcher, part of an oeuvre that also includes Shit Carcass and Necrophiled and Cannibalised.

Beryl is a character, she explains, “like a little girl who witnessed the murder of her parents by one of the other members of the band and, because she’s been horribly abused by them, joins in and becomes a serial killer. Like their protege, I suppose.”

This is underground metal, the stuff that exists and thrives in another dimension to the stadium-filling, multi-platinum-selling world of Metallica et al – which is already pretty underground to many people. Every genre has a hinterland, but none quite like heavy metal’s: a vast, vibrant, labyrinth that finds room for everything from stuff so extreme it counts as avant garde experimental music to artists painstakingly recreating the sound of long-gone bands no one outside underground metal has heard of in the first place.

It began in the mid-80s, as a DIY reaction to the increasingly commercial sound of mainstream metal. Unmoved by the kind of hard rock that was being shown on MTV, fans traded demo tapes by ever more obscure and extreme bands from around the world – and, 30 years on, something of the bedroom obsessive still clings to it. It can appear forbidding to the outsider, not so much because of the blood-and-guts imagery, but because it frequently feels deliberately unintelligible, as if trying to ward off anyone who doesn’t display sufficient commitment: the illegible band logos, the baffling array of microcosmic sub-genres. Are you a fan of technical brutal death metal, or melodic sludge metal? Do you eschew serial snuff grind in favour of pornogrind?

Facebook Twitter Pinterest Blood and guts … Basement Torture Killings’ ‘Beryl’ lets rip. Photograph: Jan Chrstn

In fairness, virtually everyone I speak to – people so entrenched in the underground scene that they understand the difference between depressive black metal and depressive suicidal black metal – seems either faintly exasperated or bemused by the proliferation of sub-genres. “I think it’s a natural response to the number of bands on the internet,” says Marek Steven, guitarist in underground metal band Amulet and promoter of Live Evil, an annual “international heavy metal festival presenting underground black metal, thrash, speed, death, heavy metal, metal punk and doom”.

Steven adds: “You can find a genre like depressive Finnish black metal and you can become an expert in the 50 bands that have played that since 1988. Everyone likes to feel belonging. If you feel a camaraderie with people that like the very specific thing you’re into, that’s fine, but it shouldn’t be elitist. There’s a bit of a genre-fication issue with underground metal. So, yeah, I would say: stop trying to be so specialised – just be open, just play music.”

Nevertheless, there are certainly areas that cloak themselves in a kind of wilful obscurity. As Marcus Mustafa – owner of London’s solitary specialist heavy metal record shop Crypt of the Wizard – puts it: “Bands want to maintain themselves as small. They’re like, ‘Don’t listen to this record, don’t talk about us.’”

He and Crypt of the Wizard’s manager Charlie Wooley reel off examples – the legendary French black metal bands of the Légions Noires collective, who refused to release any albums or play live, preferring to circulate demos in tiny numbers among their friends, which eventually leaked on to the internet; labels such as California’s Rhinocervus, which released albums and EPs without titles, artist names or track listings; festivals that decline to inform fans who’s actually playing, “so it’s like, ‘Are you strong enough to come anyway?’”

It’s an extreme ethos partly founded in a rejection of commerciality. “I think it’s a bit like if you can’t make money doing something,” says Charlie, “then you do the opposite: you try not to make any money.” But it also reflects a desire to maintain a certain mystery. “A lot of it came out of the black metal scene, which was so often about creating an atmosphere and a mystique. From wearing the corpse paint makeup to the pseudonyms, it takes away from the boring bloke-playing-a-guitar thing.”

Facebook Twitter Pinterest An extreme ethos … Charlie Wooley, left, and Marcus Mustafa at Crypt of the Wizard record shop in Hackney, London. Photograph: Antonio Olmos/The Guardian

Indeed, if you want a quick demonstration of how easily a band’s underground mystique can be punctured, consider Lapland’s Beherit, whose early recordings bore out their aim to play “the most primitive, savage, hell-obsessed metal imaginable”, whose members were called Nuclear Holocausto, Black Jesus and Sodomatic Slaughter, and who became embroiled in a conflict between Finnish and Norwegian bands known in metal lore as “the Dark War”. Now type “Beherit live 1990” into YouTube and boggle at the footage: Nuclear Holocausto, Sodomatic Slaughter and the rest look about 14 years old, and are playing outside a branch of Benetton to a crowd of visibly bored shoppers.

Legendary French collective ​Légions Noires refused to release any albums – or play live

The anonymous wing of black metal aside, debate rages over precisely how underground underground metal should be. In a Brighton coffee shop, I meet Paul Carter, veteran of the 80s tape-trading scene and guitarist and vocalist in Thus Defiled, who’ve spent a quarter of a century rigorously guarding their independence. “There’s an underground attitude that’s, ‘No gods, no masters.’ It’s all about the real grassroots belief in what you do. We define what we do at any point in time. It would become different if a label said, ‘There’s a load of money, now we want one album in six months and another in 18.’ If you can’t believe 100% in what you do, it becomes of no value.”

Marek Steven isn’t so sure. “I’m very passionately a supporter of the underground – I’ll travel to Sweden to see a tiny band that’s not signed yet because I love them. But when I was very young, heavy metal was huge. Everyone was into Saxon, Iron Maiden, Motörhead. That’s what I want again – I want everyone to enjoy this music that I love.”

Facebook Twitter Pinterest Creating an atmosphere … recordings on sale at Crypt of the Wizard. Photograph: Antonio Olmos/The Guardian

But as Charlie Wooley points out, the whole debate is usually theoretical anyway. Bands occasionally break through to the mainstream, most notably Sweden’s Ghost, who played their first ever booked show at Live Evil in 2010 and whose last album reached the US Top 10 and won a Grammy. But they’re the exceptions that prove the rule. “For most bands, being underground is not a transitional phase,” he says. “A lot of underground metal is quite hard to listen to from a beginner’s perspective. If you’ve been listening to black metal for years, then put on an underground Mexican war metal album, you’ll have some purchase to start off with, but if you’re listening to it cold for the first time, it’s just noise.”

That means life in an underground metal band is frequently a hardscrabble, hand-to-mouth existence. Whenever my interviewees describe a band as “fucking huge”, it invariably transpires that means album sales in the tens of thousands: not bad, but not enough to give up your day job. Far more are looking at selling hundreds of albums and playing to small crowds, often by exchanging gigs in their hometown with another band.

I talk to James McBain, marginally better known as Hellripper, a “one-man black/speed metal band” from Aberdeen, whose self-released album Coagulating Darkness has been picking up laudatory reviews, despite being promoted largely via social media. Aside from the fact that the album features a guest appearance by his parents on vocals, the striking thing about his career is its weird combination of vast global reach (he’s played live in Romania, his stuff has been released in Colombia, Chile and the US) and the tiny pockets of audiences it finds.

Facebook Twitter Pinterest A quarter century of grassroots dedication … Thus Defiled

“In Aberdeen, our primary venue right now is a place called Musical Vision. It’s a rehearsal room and it’s got a 40-person capacity – and it’s usually the same 40 people that turn up. Then last weekend we played in Romania and there was like 100 or 200 people there, it was a pretty daunting experience, playing with a bunch of huge bands like Necrophobic. Well,” he corrects himself, “huge bands for the underground metal scene.”

“It’s got to be a hobby,” says Marek Steven. “Most of the bands are happy just to get a record label to pay for printing their album, get a percentage of sales or get given 50 or 100 copies to sell on the road. You’re paying for rehearsals every week, you’ve got to buy equipment. Touring in the UK’s a nightmare – you’re sleeping on floors, and lucky if you sell some T-shirts and break even. It’s very DIY. People do it because they just want to play.”

One fanzine explores how the black metal aesthetic became 'entangled with the topography of northern hemisphere forests'

And that’s the thing. For all the shoestring budgets and day jobs, the complaints about the lack of venues in Britain or hipsters co-opting the scene, everyone I speak to is wildly enthusiastic about the health of underground metal. The way it keeps shifting and changing, how it encompasses a teeming multiplicity of styles, or the return of printed fanzines called things like Bardo Methodology or Becoming the Forest, the latter the work of a visual artist called Una Hamilton Helle, which explores “how the aesthetic and philosophy of black metal has become entangled with the topography of the northern hemisphere’s abundance of dense spruce forests”.

The list of great bands I’m told to listen to is endless – Live Burial, Wytch Hazel, Insurgency, De Profundis, Eliminator – while Crypt of the Wizard appears to be burgeoning. They’re launching a label to release an album by Ghold: “An experimental sludge band,” enthuses Charlie, “really heavy, a fucking difficult band.” And they have other plans for expansion. “People keep coming in and saying there’s nothing like this shop in the north,” he says. “So we’ve had this idea to build a mini, mobile record shop and take it on tour.” Do they have any particular vehicle in mind? “We’re looking at hearses at the moment.”