Frost blister
From Glossary of Meteorology
frost blister
A seasonal frost mound produced through doming of seasonally frozen ground by a subsurface accumulation of water under high hydraulic potential during progressive freezing of the active layer.
(Glossary of Permafrost and Related Ground-Ice Terms, National Research Council of Canada, NRCC 27952, Technical Memorandum No. 142, 1988.)