Grand Terre means 'big land' in French. It is the main island of New Caledonia and is known as the mainland. The island is around 400km by 50km wide making it the biggest island in the country by far.
The whole island sits within the world's largest lagoon at 23,5000 sq kms. This lagoon is possible due to the islands location within the New Caledonia Reef. This reef is the second-longest double-barrier reef on Earth at 1,500 kilometers long and only surpassed by Australia's Great Barrier Reef. Interestingly enough both the Australian and New Caledonian reefs are located in the same sea, the aptly named Coral Sea.