Back in the days when Bankside Power Station actually burned oil, it stored its fuel supply in a trio of enormous round tanks lying beneath the rear of the building. Nobody can afford such a quantity of oil these days, but the tanks are still down there, and Tate Modern has big plans for them. Last weekend, small groups of museum members toured the eerie underbelly of the institution for a look at things to come.

While one of the tanks will remain sealed off for sinister-sounding "back-of-house operations," the other two are to become, respectively, a display space and a performance space. They are odd and cavernous enclosures to be employing on both counts, and hopefully the curators will be able to use this oddness to good effect. The display space will provide a home for works that are too small for the Turbine Hall and too big for anywhere else - think Joseph Beuys's Lightning with Stag in its Glare and then some.

What's more, this is only the first step in a long-brewing £18 million plan called Transforming Tate Modern (power station pun allegedly unintended). Eventually - meaning, of course, in 2012 - a new eleven-storey tower of art will rise above the tanks, giving the impression that the Tate has run up against an iceberg. Here's hoping that image doesn't hold too much symbolic weight. Planning permission is already in place, so we're just relieved that the new wing is lower-key than the plans for Battersea.

The tanks themselves are pure industrial functionalism, untouched by the imaginative hand of Sir Giles Gilbert Scott, but they are a unique corner of our always fascinating subterranean
London. If you didn't do the tour last weekend, you can have a virtual look but you have to imagine for yourself the resonant sonic environment, heady industrial smells and sheer scale. Better yet, book early for 27 September when a mysteryperformance/installation will launch the tanks' artistic careers.

It's definitely worth getting a look down there before the rust stains and exposed wiring are covered over with gallery white.

By Paul Cox

Photos by Amanda Farah.