The Tate Modern extension by Swiss architects Herzog & de Meuron is one of the most hotly anticipated architecture projects in London - the Olympic Park aside. Even though the project probably won't see its full completion until 2016, phase one has just finished, with the striking underground Tate Tanks opening to the public this week, dedicated to live art, performance, installation and film.

'Today we are opening the next phase in the evolution of the Tate', director Nicholas Serota announced at the launch. He highlighted the project's 'scale, ambition, dedication and consistency'.

But visitors expecting architectural spectacles and trickery may be disappointed. Instead the three large circular galleries, in what used to be the former power station's oil tanks, are most striking for their concrete nakedness. Herzog & de Meuron have kept the spaces as close to their original identity as possible, creating stark and imposing volumes that will act as a foil to the live artworks on show.

Overall, the large-scale work-in-progress Tate extension - developing right beneath our noses at Wallpaper* HQ - poses a unique challenge to Herzog & de Meuron, who were also responsible for the original Tate Modern building 12 years ago. 'The Tate is a very public forum,' says Jacques Herzog. 'Our role as architects is to make it function as a whole and feel natural, not fragmented'.

Lighting design by Arup Lighting, as well as an overall design language that references the original power station structure, ensure a seamless transition from the Turbine Hall to the Tanks.

The ambitious 15-week opening programme at the Tate Tanks comprises everything from a specially commissioned new installation by Korean artist Sung Hwan Kim, to dance and performance pieces by the likes of important choreographer Anne Teresa de Keersmaeker. In these new subterranean spaces, art and architecture will come together in the most engaging and thought-provoking fashion.