Lung cancer. Coloured scanning electron micro- graph (SEM) of a small cancerous tumour (blue) filling an alveolus of the human lung. Alveoli are the blind-ended air sacs which make up the lungs. Here, the individual cancer cells are coated with microscopic, hair-like structures known as microvilli. A number of cancer cells can also be seen separated from the main tumour. The most common cause of lung cancer is cigarette smoking. Magnification: x300 at 6x7cm size.