STEREO-B saw this big filament from its point of view looking over at back-side of the Sun.

The filament is the large dark straggly line of material in the upper left of the movie. Click here to see the movie in helioviewer.org.

Filaments are cool strands of material about 100 times cooler than the surrounding plasma, and are supported by magnetic fields. They can lie suspended but cool in the hot solar atmosphere for weeks, and then erupt in a matter of minutes, causing coronal mass ejections. It should be rotating into the SDO field of view in the next couple of days. If you are on Facebook, The Sun Today has a great post about this filament. Let’s see what happens!