It's costing tens of millions of pounds and will take another year to be completed.
A major project to refurbish and strengthen the famous road tunnels leading to Heathrow airport is continuing.
It's to improve safety and allow for the new generation of heavier planes using the runway above.
The work is being carried out during the night so the airport can operate as normal during the day.
Our transport correspondent Mike Pearse has more.
Tens of millions of pounds is being spent giving the famous road tunnels into Heathrow a makeover. The tunnels, off the M4, were built in the 1950's and are used by 55,000 vehicles a day. The work is aimed at improving safety and to allow a new generation of heavier planes to travel on the runway which goes over them.
The two 630 metre long tunnels are being refurbished overnight to allow the airport to operate as normal during the day. New fire systems are being installed and are being made stronger to cope with larger planes like the A380.
Every night at 1030 one of the two tunnels is closed with the other a contra-flow to keep the airport operational, before an army of workers and vehicles move in. To keep disruption to a minimum the tunnel has to reopen at 5.30 in the morning. The two year project is now at the half way stage. Work will continue five nights a week for the next year. It's hoped at the end of the tunnels will be fit for the next thirty years.
Campaigners have gathered at Uxbridge Magistrates in support of 13 protesters who staged a demonstration at Heathrow in July.
It led to 22 flights being cancelled, with delays for thousands of passengers.
The protesters face charges of trespass and entering a secure area.
They pleaded not guilty and will face a two-week trial at a later date.
A major conference is underway on the recommendation to expand Heathrow Airport rather than Gatwick. Runways UK - which promotes airport expansion is examining the best use of existing resources before any new runway is built..
As our Transport Correspondent Mike Pearse reports both major airports say they need to expand. And he's been looking at the impact of new runways at an airport in the USA which is more than three times the size of Heathrow.
Campaigners opposed to a third runway at Heathrow have been at Westminster in a final attempt to persuade David Cameron to reject the plan. They packed a suitcase full of books and DVDs against airport expansion for him to read on his summer holiday.
Members of Parliament from the south given their reaction to a report recommending Heathrow is given the go-ahead to expand.
The findings were published by the Airport Commission which took three years to reach its conclusion which looked at the best way to increase the country's airport capacity.
Reaction below from: Phillip Lee (Bracknell), Adam Afriyie (Windsor), Boris Johnson (Mayor of London), Harriet Harman (acting Labour leader), Caroline Lucas (Brighton Pavilion), Nick Herbert (Arundel and South Downs), Henry Smith (Crawley), Carwyn Cox (Windsor and Maidenhead Council), Ranil Jayawardena (North East Hampshire) and Patrick Mcloughlin (Transport Secretary).
Members of Parliament from the south east have expressed their reaction to a report saying a third runway at Heathrow is the best option to increase the country's airport capacity.
The findings were published by the Airport Commission which took three years to reach its conclusion.
Reaction from: Henry Smith (Crawley), Jeremy Quinn (Horsham), Tom Tugendhat (Tonbridge and Malling), Caroline Ansell (Eastbourne), Nus Ghani (Wealden), Adam Afriyie (Windsor) and Kelly Tolhurst (Rochester and Strood)
The Mayor of London, Boris Johnson, said today that a new runway for Heathrow would mean a massive increase in noise pollution and fumes.
David Cameron announced a short time ago that a decision on a new runway for the region would be made by the end of the year. The recommended option, announced today, is Heathrow.
Members of the Gatwick Area Conservation Campaign have been celebrating after today's announcement. Heathrow is the preferred option for a new runway. The interviewee is Sally Pavey.