In the distant past, the Earth’s climate was a lot different that now. Sometimes it’s been a lot warmer than it is now—warm enough for trees to grow right up to the north and south poles. Sometimes it’s been a lot colder than now, with ice covering most of the planet. Over thousands and millions of years, climate has changed many times.
During ice ages, the water level in the oceans has dropped, because so much water was frozen on land as ice and snow. In the most recent ice age, most of the United States and Canada was covered in sheets of ice called glaciers. This was not that long ago, at least compared to the age of the Earth—the ice age ended about 12,000 years ago.
The water level in the oceans has gotten higher during warm periods. That’s because much of the ice and snow on the land melted.