A mass rapid transit (MRT) railway system is soon to be constructed in Singapore. Attention is being paid to details of the engineering geology of the island, which are generally not well documented or understood. The main solid formations, both igneous and sedimentary, are deeply weathered, often with an abrupt boundary between completely weathered and slightly to moderately weathered rocks. They have a high degree of fracturing and the sedimentary Jurong Formation has a high but variable dip. They are overlain by Quaternary deposits, many of which have poor geotechnical properties. A soft marine clay and overlying peaty deposits present particular problems in excavations. Each has a low strength and high compressibility and water tables are close to the ground surface. Variations in the geotechnical characteristics of the marine clay are now being recognized. The soft deposits are mainly found in buried channels cut during lower stands of sea level in the recent geological past. The channels have steep sides, and at many sites geotechnical conditions change very rapidly.