In the 1595 Map of the Arctic, Mercator shows the North Pole as having a substantial landmass surrounding a rock.
A little over a hundred years later, Guillaume de L'Isle's 1714 map (updated 1741) of the Arctic regions shows no land beneath the polar ice cap.
Apparently, Guillaume de L'Isle was not the only person to believe there was no substantial landmass under the Arctic ice cap. Wikipedia's article on the North Pole states:
As early as the 16th century, many prominent people correctly believed that the North Pole was in a sea
In the absence of solid proof provided by modern scientific techniques such as bathymetric measurements of the sea floor, what led de L'Isle and others to believe there was no land under the Arctic ice cap?