_:b670492659 . _:b670492659 . _:b670492659 . _:b670492659 "A layer which is composed of ground that is 1) subject to annual thawing and freezing and 2) present in areas underlain by permafrost." . _:b670492659 "https://nsidc.org/cryosphere/glossary/term/active-layer" . _:b670492659 .