Complex sentence: The vents were discovered in December 2000 during a National Science Foundation expedition to the mid-Atlantic, a second expedition mounted in 2003 used the Alvin submersible to explore the vents.
Split into simpler sentences: The vents were discovered in December 2000 during a National Science Foundation expedition to the mid-Atlantic. A second expedition mounted in 2003 used the Alvin submersible to explore the vents.