Chernobyl?or maybe a nuclear bomb.
anyway once singapore become deserted,the infrastructure will fall apart,once the drainage system start failing,coupled with the high rain fall,singapore should be a swamp again.
if u play the game metro 2033 which takes place after a nuclear holocaust in russia(no humans left on earth,only 30,000 underground) u will notice besides the ruins of cities,many parts of the cities turned into swamps and mangrooves and jungles.