Other forms: sea levels
Sea level is a scale for measuring the height of an area of land, using the average height of the surface of one or more oceans. The city of Amsterdam is actually two meters below sea level.
If you've ever watched the tide go out at the beach, you know that the actual height of the sea varies widely — and it changes over time, too. Sea levels are higher today than they used to be because of climate change. To determine what sea level is, scientists measure at different times and determine an average, sometimes also averaging the heights of several oceans. The tallest peaks in Rocky Mountains are more than 14,000 feet above sea level.