What is going on? "Ballroom Blitz", that's what. This is all part of an operation that has involved a level of planning that would be adequate for a takeover bid in the City or a small war in some far-flung corner of the globe. Over the next two weeks, this vast central space will be leapt on, danced over, stamped upon and rolled around by over 20,000 people who every year turn up eagerly at an event that started out as a "why not" sort of idea that just grew and grew.
Why not expand the horizons of the people who come to the Festival Hall regularly to see ballet? Why not fill the foyers with free dance classes, workshops, demonstrations and performances? Why not focus it on the Festival Hall's vast 1950s ballroom and call it "Ballroom Blitz"?
This year's season runs until 13 August - the day taken over by the Peggy Spencer team of formation dancers from Penge - with a great plethora of dance of all sorts in between.
"It's a nightmare," confides Lester Sayer, a chunky cheerful man who, with seven Blitzes behind him, is responsible for technical equipment - from half a mile of cables to dancefloors and rostrums. But it's a nightmare that inspires devotion in its team of backroom workers.
The event began on Sunday when the Appalachian Clog Dancers drew 80 people on to the dancefloor; and in a crowded office high in the centre, Alistair Spalding, performing arts producer, studies a computer screen and attempts to predict and organise the minutest and many needs of each future event. There are four large-scale "social dances" - from ceilidh to salsa - plus a piece drawing on the West African "griot" storytelling traditon, created specially for the event by choreographer "H" Patten and accompanied by griot musician Malamini Jobarteh and the spontaneous visual artistry of Emanuel Jegede and Clary Salandy. Open Yu Mouth Tory Jump Out climaxes "Ballroom Blitz" on 11 August.
Meanwhile, down in the foyer, Fiona Edwards and five sinewy dancers tentatively try out their new work. Inspired by the 1950s, it includes a youth dance group from Essex who are still to be slotted in. "And a live jazz band," says Edwards, with an edge of anxiety. A tight budget means that they can't rehearse with the band in advance. "I just hope that's going to work."
The floor is just right for dancing, though. "A team of cleaners were up all night swabbing and rinsing the space," says site manager Tony Green. They wash it down with water first, and then with a mixture of stripper and fresh water to make it less slippy - don't want any torn tendons during the tango, do we?
Early every morning they'll be there, sweeping and washing from 4am. In between they deal with the rubbish: about 60 bags will go out during the day to the three gigantic skips waiting below. Does Mr Green enjoy it? "Well, yes. You get more kind of audience participation - if that's the right thing to call it. It's a good thing."
"It's more than a good thing," says Sayer expansively. "We don't have much to do with Joe Public usually. We just put things on for them. Here, we're all on the floor together. That's where you get the buzz of it. At the end of the day, you sit back and say, 'Yeah, that was good, that worked.' "
To 13 August. Details: 0171-928 8800