Final answer:
The Mercalli scale's highest damage designation is XII or 12, which indicates total destruction and possible significant alterations to the earth's surface.
Step-by-step explanation:
The maximum possible damage designation on the Mercalli scale is XII or 12. This scale, formally known as the Modified Mercalli Intensity Scale, is used to measure the intensity of an earthquake based on its observed effects to people, buildings, and the natural environment, rather than the energy released at the source, which is measured by the Richter or moment magnitude scales. The level XII on the Mercalli scale indicates total destruction to structures, and the earth's surface may be significantly altered.