Maui Nui ("large Maui" in Hawai'ian) is the name given to a prehistoric island that existed in the Hawaiian Island chain over a million years ago. Maui Nui was made up of four existing islands (Maui, Moloka'i, Lana'i, and Kaho'olawe) and the seafloor between them. At its maximum size, Maui Nui was larger than the current Big Island (Hawai'i), reaching 160 miles from west to east.
Seven volcanoes grew to form the island, as the Pacific plate drifted over the volcanic hot spot. The island grew from west to east, beginning with the Penguin Bank volcano nearly two million years ago. As the plate drifted, additional volcanoes were created: West Moloka'i, East Moloka'i, Lana'i, West Maui, Kaho'olawe. The most recent of these volcanoes, Haleakala, is the only one which is still active.
As those volcanoes became dormant, the island subsided, allowing the ocean to encroach into the spaces in-between. Additional under sea land slides and rising sea levels after the last ice age also contributed to the disappearance of the land mass.
Currently, the sea floor in the areas between Maui, Moloka'i, Lana'i, and Kaho'olawe are very shallow compared to the surrounding sea floor. At most, the ocean depth reaches merely 500 meters between the islands, compared to several thousand meters in the surrounding area. The area between East Maui and Haleakala is still above sea level, and makes up the "saddle" of the island of Maui.
Resources:
http://hvo.wr.usgs.gov/volcanowatch/2003/03_04_10.html
http://hvo.wr.usgs.gov/volcanowatch/1995/95_09_22.html
http://www.mbari.org/education/internship/07interns/papers/InternPapers/IFaichney.pdf