What was the last unexplored frontier?

The last unexplored frontier is often considered to be the deep ocean, which covers over 70% of the Earth's surface. Despite advances in technology, more than 80% of the ocean remains unmapped and unexplored, harboring unknown species and geological features. This vast underwater realm presents significant challenges for exploration due to extreme conditions, depth, and accessibility. Another perspective views outer space, particularly regions beyond our solar system, as the final frontier yet to be fully explored.