You have most probably heard of the High Line park in New York - an urban green space on top of a stretch of unused railway. But did you also know that the Landscape Institute recently held a competition in collaboration with Mayor of London and Garden Museum to find an innovative design for a similarly unique green space in London?

The winner - Pop Down by Fletcher Priest Architects - was announced on Monday, 8 October as part of the High Line symposium, a three-day event on green urban spaces. Pop Down was chosen from 20 shortlisted entries and envisages a development of an urban mushroom garden at the Mail Rail tunnels underneath London's Oxford Street. Sculptural glass-fibre mushrooms at the street level would act as a light source for the garden beneath.

Fletcher Priest Architects commented: 'We are actively looking at the next steps for proposal. It is a very exciting opportunity to explore the possibility of opening up spectacular hidden spaces.'

The judging panel, which included the co-founders of High Line park, Joshua David and Robert Hammond, were so impressed with the quality of shortlisted entries that they decided to also award a runner-up title as well as three highly commended places.

The runner up was [Y/N] Studio's LidoLine idea, which envisions a swimmable channel from Little Venice to Limehouse in the Regent's Canal.

Other entries marked highly commended were Bridge-It from HTA, an idea that plans to unlock inaccessible transport corridors around the existing London public transport network and Barge Walk by Erika Richmond and Peggy Pei-Chi Chi, with designs for a floating park and wetland barges at the edge of Canary Wharf. The third highly commended place went to Bus Roots by Wynne James, who proposed to create small gardens on top of bus shelters.