What is the world's deepest known point below sea level on land under ice?

The world's deepest known point below sea level on land beneath ice is beneath Denman Glacier in Antarctica.

Bedrock under the glacier reaches about 3,500 metres below sea level, according to mapping by the BedMachine Antarctica project. The extreme depth lies in a deep subglacial canyon hidden beneath the ice in Queen Mary Land, rather than in an exposed mountain valley or ocean trench.

Denman Glacier flows northward for roughly 70 miles before reaching the Shackleton Ice Shelf. It was identified during the Australasian Antarctic Expedition in 1912 and named for Lord Denman, then Governor-General of Australia.

The wording can cause confusion because the deepest feature is technically the canyon or bedrock beneath Denman Glacier, not the moving ice itself. Other Antarctic subglacial features, including the Bentley Subglacial Trench, are also extraordinarily deep, but current mapping places the Denman bedrock depression lower than them.