The challenge with the coastal hypothesis is the same one that confronts any hypothesis about events that took place along coastlines during the Ice Age: the coast of 15,000 to 20,000 years ago is now underwater.

Sea levels were 120 to 130 meters lower during the glacial maximum than they are today, and the rise in sea level as the glaciers melted — a process that unfolded over several thousand years — submerged the coastal landscapes where the earliest coastal Americans would have camped, fished, and died.

Sites that would directly document a coastal migration route are therefore largely inaccessible to standard archaeological survey.

Some have been found by underwater archaeologists working along the submerged continental shelf, and the evidence from these sites is consistent with early coastal occupation — but the recovery is difficult and the evidence sparse.