Sea levels worldwide were approximately 300 to 400 feet lower than today.

The coastline of North America was many miles farther out than the current shoreline.

If people were living along the coasts of North America 23,000 years ago — and the coastal migration model suggests they were — their camps and settlements were on land that is now under hundreds of feet of ocean water.

Finding those sites is not impossible, but it requires deep-water archaeology, sophisticated search methods, and significant resources.

Relatively little such work has been done, because the logistics and costs are daunting.

The same logic applies to the interior.