The current climate warming in the eastern coastal area of Chukotka contributes to deeper seasonal thawing and activation of thermal erosional landslides, leading to the exposure of massive ice bodies. The thawing of exposed massive ice courses the coastline destruction and settlement infrastructure damage. The study of massive ice bodies on the eastern coast of Chukotka, including radiocarbon AMS dating and determination of ice isotopic composition, showed that most of the studied ice bodies were formed in the Late Pleistocene, between 22 and 27.5 thousand calibrated years ago. Relatively high values of the ice isotopic composition and significant variations of the δ18O values of up to 6‰, indicate a predominantly ground origin of massive ice bodies, the formation of which occurred during the freezing of water-saturated sediments in a closed system. The source for the massive ice formation could be water-saturated sediments of sub-lake taliks or supra-permafrost ground-water. However, it is possible that locally massive ice bodies or their peripheral parts could be formed as a result of burial of snow patches or floating ice in the Holocene that may be indicated by a fairly uniform distribution of isotope values and the similarity of isotopic composition of some massive ice bodies and Holocene ice wedges. © 2025 Elsevier B.V., All rights reserved.