Complex sentence: Sir Martin Frobisher was the first of a long line of explorers to venture the Northwest Passage; but it was Henry Hudson who discovered the gateway to the Northwest (Hudson Bay) in 1610.
Split into simpler sentences: Sir Martin Frobisher was the first of a long line of explorers to venture the Northwest Passage. Although, it was Henry Hudson that discovered the gateway to the Northwest (Hudson Bay) in 1610.