The wind has a vocabulary here: it scours the tongue of the glacier, lifts powdered snow in fine sheets, and whispers through a doorway of blue that seems to have been carved by another planet. On the Premium Ice Cave Tour at Vatnajökull, travelers step from a warm van into a landscape that is at once brutal and strangely domestic — black lobes of volcanic rock, skeletal outcrops, and an ice sheet that advances and retreats like breath. Guided by professionals, small groups cross the rimmed moraine toward Breiðamerkurjökull, a glacier arm that harbors the ephemeral blue caverns beneath Vatnajökull, Europe’s largest ice cap.