Some parts of Hokkaido are still in their natural state.
Literal
Hokkaido [of-の] one-part [topic-は] still nature [of-の] as-is [of-の] state [in-で] remaining-is.
自然のまま ('as nature is, in its natural state') uses まま ('as is, unchanged'). 一部分 ('one part, a portion'). 残る is the intransitive 'to remain, to be left behind.' A nod to Hokkaido's relative wilderness compared to the rest of Japan.