AN underground movement is gaining support in Singapore. No, the republic is not being overrun with political subversives - just too many people. Government planners have been exploring means for more creative land use, having revised their long-term population projection from four million to more than five million. Singapore is already building up into the air and out into the sea. Now it is looking underground. The Defence Ministry, which uses 20 per cent of Singapore's 647 square km, will lead the way with three rock caverns to serve as ammunition depots. Excavation of the caverns, 100 metres underground in a disused quarry in Mandai, should be completed by 2003. The Defence Ministry hopes to free up 400 hectares, both through underground construction and building multi-storey facilities. This is just the start. The Government is studying the feasibility of constructing large-scale oil and gas stores in the Jurong industrial area and possibly some outlying islands. A four-year study by Nanyang Technological University (NTU) concluded that caverns large enough to hold 100,000 cubic metres of propane, could be built near Pandan Reservoir. Other projects studied include a warehouse complex of five caverns with 210,000 square metres of storage space carved out of the side of Mount Faber, and multi-purpose caverns cut into hills at NTU's own campus providing 30,000 square metres for car parks, offices, laboratories or an auditorium. The biggest proposal of all is also the most hush-hush. A feasibility study is believed to be under way into building an Underground Science City. Less fanciful are proposals to develop a deep tunnel sewerage system. Researchers claim cavern space costs on average S$300-$400 (HK$1,390-$1,850) per cubic metre to construct. 'When the cost of the land is considered, cavern development costs are lower than above-ground developments,' NTU associate professor Zhao Jian said.