Hundreds are expected to apply to visit the famous Crystal Palace Subway after a ballot for places opened this month.
Last year marked the first time the Grade II vaulted walkway opened to the public as part of a London-wide cultural event.
More than 1,500 took part in the ballot in 2013, with just 500 granted supervised access to see the Victorian arched brick work and hidden walkways.
Chair of the Friends of Crystal Palace subway Karl Richter said: "Last year’s visitors were hugely enthusiastic and gave us very positive feedback."
However, while Mr Richter added an extra 260 will be able to visit this year, numbers have to be limited – so competition for places will be fierce.
The 'Open House' weekend, over September 20-21, will also see a free volunteer-led exhibition entitled 'Inspired by the Subway', capturing some of the stories of the people who walked its underground passages.
Project manager Sue Giovanni said: "Working with such an enthusiastic and capable group of volunteers has been the perfect way to investigate the creative uses the local community has put the subway to since the fall of the Crystal Palace in 1936.
"The project has benefitted enormously from the variety of skills and diversity of approaches that the volunteers have brought to it."
The subway was opened in 1865 to create access to the Crystal Palace High Level Station.
However, with the station closing in 1956 following the famous Crystal Palace fire in 1936, it was all but forgotten and used only as an air raid shelter in the Second World War, and later as an unofficial children's playground - until it was fenced off completely.
In recent years the subway has been closed due to safety concerns, but the Friends of Crystal Palace Subway now aims to reopen the space for more community use.
The weekend will also commemorate the 60th anniversary of when the last train left Crystal Palace High Level Station.
For more information about how to apply to the ballot, which closes on Sunday August 31, visit cpsubway.org.uk