In this color-enhanced scanning electron micrograph, a melanoma cell (purple) passes through the skin's basement membrane, possibly with the help of digestive enzymes called proteases. 

In their normal state, many cells are sedentary and tightly held in place by molecular bonds that lock one cell into another and lock the cells into connective tissue.

The ability to either secrete proteases themselves or to induce surrounding cells to secrete proteases is just one of the many changes that turns normal cells into cancer cells, and cancer cells into metastatic cancer cells.