Bringing together dronelords Sunn O))) and black metal survivors and experimental kings Ulver, Terrestrials has been trailed for a while as something truly immense, that solar artwork suggesting that heavenly-body levels of grandiosity awaited. Although it's not the first time that members of these two bands have collaborated (O'Malley and Rygg play together in ambient drone project Aethenor and the bands worked together on a bonus track for Sunn O)))'s White One) it is the first time that these two giants of the atmospheric music world have collided in such epic fashion for a full-length release. Yet the closer you look, the more the hype fades – Terrestrials is based on recordings made over a single night all the way back in 2008 (before Sunn O)))'s almighty Monoliths & Dimensions and just after Ulver's Shadows of the Sun!) that was subsequently worked on in-studio, including the addition of trumpet and violin. It's hard to hear about artists coming together and creating spontaneous magic, and then (perhaps naively) realising that the magic is far from spontaneous, and there seems little explanation for why this half-hour or so of music has taken six years to be released...

Indeed, it's hard not to be disappointed with Terrestrials after initial listens. It doesn't have that feeling of being born of a black star, as the best Sunn O))) material does, yet seems far more weighted towards the Americans. Each of the three nine-minute plus tracks here follows much the same route of slowly building ambient drone. This is exactly what you'd expect to hear from such a collaboration; if I wanted to damn it with faint praise, I'd call it pleasant, something that drone and ambient of the underground metal variety rarely see as a goal. Nothing here is outside of the comfort zones that Ulver and Sunn O))) have dug out for themselves in these modern times when black metal and drone aren't automatic insults from mainstream music fans. Yet give it time, and Terrestrials is a grower. Unlike Ulver's most recent album Messe I.X – VI.X, it doesn't feel dull or particularly difficult to connect with – the primordial hum that Sunn O))) have brought to the table gives this a warmth that's far less fragile than Ulver and far more human than the Americans normally are. Opener Let There Be Light grows from silence, pulsating warmly and growing in size and stature as the eleven-minute track progresses; it's an exact musical depiction of sunrise, with subtle wordless vocals from Rygg blending in perfectly. The trumpet trills slowly and lengthily, more jazz than metal yet giving an epic flourish to the music, and a few brief minutes of percussive heaviness makes for the perfect accompaniment, giving the feeling of a destination that this sort of music can all too often neglect in favour of the journey.

Sadly, the following Western Horn is an example of this. Building up again, the track features plenty of the deep hums and shimmering fuzz that we know and love from Sunn O))), but seems to be treading water, doing little to make itself memorable or even prove it has life. Fortunately, closing fourteen-minuter Eternal Return is much better, a slow riff providing a distant counterpoint to the upfront keyboard meandering – we even get Kristoffer Rygg's slow, deep voice singing about seven minutes in, atop a simple yet fitting synthesizer trill. He proves some much-needed emotional input before the track peaks, a noisy squall signalling the end and a fade out. It's not the best you've heard from either Ulver or Sunn O))), and tempered expectations are probably best if you're the sort of fan that expects boundaries to be constantly challenged. Hard not to be with these particular ingredients, I know; both bands have enough depth and breadth to their back catalogues to make an almost limitless array of what-ifs, from Nattens Madrigal and White One teaming up for a cruel noise extravaganza to a folky Bergtatt-flavoured darkness on Monoliths & Dimensions. I'd like to hear something outside of expectations coming from two bands that once pushed boundaries but now seem content in a very confined (if very refined) area of ambient experimentation. Yet those who have aged with the bands will find much to appreciate, even if it does suggest that collaboration can mean treading water in mutual comfort zones. Hear (and buy) Terrestrials at Bandcamp.