Thwaites Glacier (about the size of Florida) is called the most dangerous glacier in the world, having caused almost 4% of sea level rise in the past quarter century. Scientists suspected it wasn't tightly attached to the bedrock, but were astounded when ice-penetrating radar revealed that warm ocean waters had eaten away a massive cavity two-thirds the size of Manhattan from its underside (shown in red below). The black line that moves further back is Thwaites' grounding line.
... the cavity would once have been large enough to hold some 14 billion tonnes of ice. Even more disturbing, the researchers say it lost most of this ice volume over the last three years alone.
The Thwaites Glacier actually holds in neighbouring glaciers and ice masses further inland. If its buttressing force disappeared, the consequences could be unthinkable, which is why it's considered such a pivotal natural structure in the Antarctic landscape.