Lung cancer. Scanning electron micrograph (SEM) of mesothelioma cancer cells. Mesothelioma is a malignant (cancerous) lung tumour usually associated with asbestos exposure. Mesotheliomas develop on the on the pleural membrane outside the lungs, eventually growing to form a white mass that covers both lungs and obliterates the pleural cavity (space around lungs). This form of cancer is difficult to treat and is often fatal. Cancers are malignant growths that result from abnormal and uncontrolled cell division. Magnification: x3000 when printed at 10 centimetres wide. Specimen courtesy of Professor Greg Towers, University College London, UK.