How does Beowulf manage to kill Grendel's mother?

Beowulf goes to the lake where his men have been killed and goes into the water. When he reaches the bottom of the lake, Grendel's mother is waiting for him. She reaches for him, but his armor protects him. He tries to cut her, but his sword can't cut her. The two begin to wrestle, but neither gains the upper hand in this combat. Beowulf spies a large sword nearby. He manages to grab it, and in one mighty blow, he beheads Grendel's mother. Light enters the murky water then. Beowulf is still angry, however, so he also beheads Grendel, who lies dead in the cave.