Mt Erebus is a well-studied 12,500' active volcano at the heart of Ross Island. McMurdo is located at the tip of a peninsula off the island (see maps in "Getting Oriented" gallery).
Erebus is unusual is that it open, so pressure does not build up leading to major violent eruptions. The crater at the top measures about 1000' around and another 1000' deep. At the bottom lies a lava lake, about 125' across, that burps and steams and this past year has started throwing out lava bombs quite frequently in small eruptions. Erebus is also known for unusual feldspar crystals ("Erebus crystals"), and there are caves on the caldera from where steam has escaped through cracks, melting the snow/ice above. The caves create perfect conditions for the growth of a variety of stunning ice crystal forms.