There are those who lament the passing of the old NMM, as the echoes of Empire grow ever fainter in its corridors. Its director, Richard Ormond, has been accused of revisionism, of denying Britain's glorious history. For years its exhibitions have been getting more populist, with those who seek authentic artefacts increasingly finding interactive displays instead. And now, the NMM has gone all the way. Nicely in time for the floods of visitors expected in Greenwich for the Dome, it has produced a rival architectural experience to the white big top, a mile down river.

Greenwich, it hardly needs saying, stands high on the scale of historically sensitive places. It is a World Heritage Site, along with the Grand Canyon and the pyramids. The Cutty Sark is here, and the old Royal Observatory, straddling the Meridian line. In front of the NMM is the Royal Naval College by Wren, Hawksmoor and others. Predating that, to one side, is the Queen's House by Inigo Jones. The museum itself is a Regency neo-classical quadrangle, originally built as a naval school. Difficult, you might think, to do anything architecturally daring here.

But the new architecture manages to be both radical and well-nigh invisible from the outside. Inside, it is a different story: the previously unregarded Neptune Court, cluttered with piecemeal additions, is now cleared and covered with a virtuoso glass roof supported on a steel latticework, right the way across. There is not a single column to interrupt the span. Beneath this, a great podium contains a sequence of new galleries, and acts as a raised public square. On it you will see a big, semi-abstract display entitled "The Future of the Sea", that is all to do with the marine environment and man's influence on it. Jolly Jack Tars are conspicuously absent.

Two sets of architects are involved, and there's been some bitching over the credits, let me tell you. "Rick Mather Architects and the Building Design Partnership" is the official line. They are very different creatures. Mather is an American-born, London-based architect who originally made his name with some exquisite house and restaurant interiors and who has now - while keeping his practice fairly small - moved into the cultural big league. By contrast, Building Design Partnership (BDP) is a very large and chameleonic firm tackling everything from huge shopping centres to the lottery-funded revamp of the Albert Hall. It has a collaborative tradition, sharing the credits with Jeremy Dixon and Ed Jones, for instance, on the redesign of the Royal Opera House.