why subglacial Lake Ellsworth?

Lake Ellsworth, locked deep in the ice for hundreds of thousands of years is the ideal candidate for exploring the unknown world beneath. Science teams expect to find life forms that are adapted to living in its unique and isolated environment, and the lake-floor sediments are expected to reveal a record of ice and climate change.

small and therefore easy to understand
near an ice divide where lake access is not complicated by ice flow
enclosed topographically, and therefore resistant to ice sheet changes that might occur over glacial cycles
close to the logistical hub at Union Glacier, from which heavy loads can be input from South America to Antarctica and deployed to the lake site

ideal because the lake is

Science Programme Director, Professor Martin Siegert, from University of Edinburgh, explains why the lake is special