Why this is so weird
For most vertebrates, widespread ice formation would cause catastrophic cell damage. Wood frogs survive by rapidly mobilizing large amounts of glucose from the liver. That sugar acts as a cryoprotectant: ice can form around cells while the cells themselves are better protected from freezing and dehydration.
What is actually happening?
The frog does not simply become a solid block of ice in every microscopic compartment. NPS descriptions distinguish ice forming in body spaces and around cells from the protected interiors of cells. Core organs such as the heart and liver freeze later and thaw earlier, helping preserve the sequence needed for recovery.
The detail that makes it even better
Alaskan wood frogs take the feat even further. Denali National Park reports survival at temperatures as low as about −12 °C. Because they can overwinter close to the surface under leaf litter instead of retreating to deep water, they can also become active early in spring when temporary breeding pools appear.
Sources
U.S. National Park Service — Denali: Amphibians
U.S. National Park Service — Kobuk Valley: Tiny Masters of Arctic Survival
U.S. National Park Service — Gates of the Arctic: Biological Miracle