It earned the briefest of mentions in a budget speech three months ago that was soon beset by controversy and a U-turn over cash giveaways. But yesterday the government put flesh on a proposal to drill holes - big ones - beneath parts of the city in which it can bury unpopular utilities such as sewage treatment plants, fuel storage depots, refuse transfer stations and columbariums. The move could free up more than 100 hectares of valuable land for development. Two-thirds of Hong Kong sits on granite and volcanic rock suitable for hollowing out to make caverns, according to a study by the Civil Engineering and Development Department. Five suitable areas have been identified: Lam Tei in Tuen Mun, Shek Mun in Sha Tin, Siu Ho Wan on Lantau Island, Mount Davis on Hong Kong Island and Lion Rock in Kowloon. Each could house more than 20 hectares of caverns. In another, potentially controversial proposal, it plans to reclaim land at seven sites away from Victoria Harbour, with work to start in 2016. Potential sites in Tuen Mun, Tsing Yi and on Lantau and some smaller islands have been identified. Planning consultant Chan Kim-on warned that caverns lacked basic infrastructure and would require extra energy for ventilation, while reclamation could destroy natural shorelines and should be a last resort. The department has identified more than 400 facilities that could be moved. The cost of moving facilities into the caverns would be more than recouped by the sale of the land, the Development Bureau said. Permanent Secretary for Development Wai Chi-sing cited the example of moving the Sha Tin sewage treatment plant, which would cost about HK$10 billion; but the resulting 28 hectares of land with Tolo Harbour views could be sold for much more. The land, Wai said, would provide space to develop the six new pillar industries on which, Chief Executive Donald Tsang Yam-kuen said in his 2009 policy address, Hong Kong could build its future prosperity: education; medical services; environmental industries; innovation and technology; cultural and creative industries; and food safety and product testing. 'By investigating the feasibility of these proposals, we will be able to provide land in a sustainable, flexible and stable manner,' Wai said.