Rashi assumes that at the end of the Mabul, the water subsided uniformly, one Amah every four days, until after 60 days the mountain tops were exposed. Ramban disagrees, for at this rate, the land would not be dry for years! (There are mountains 10,000 Amos tall.) Moshav Zekenim (to 7:12) answers that below the mountain tops, the mountains decrease the space for water, so the water receded more quickly after clearing the mountaintops (also refer to 8:4:2:2). (To answer the question, he must say that mountains occupy more than 99% of the area above the land. Nowadays, mountains occupy a small fraction of the area! - PF)