11.   Part of the reason is that they are covered by ice for much of the year.

12.   At the time of the first migration, ice covered much of the northern world, causing the worldwide sea level to drop by hundreds of feet.

13.   Lentini said most of the lake is covered by ice, but that the ice was unstable and shifting.

14.   The area is covered by ice about half of the year.

15.   The water is shallow and rocky, and ice covers the Gulf of Finland in winter months.