The lava rivers that flow deep beneath Bal-Kriav's surface are commonly known as the Great Vein of Surtur, Surturs Artery and other glorification of the god Surtur. Writings in the Scrolls of Dawn say that in the Creation War (1486 DE - 8777 DE), Surtur was a primordial lord bent on manipulating Bal-Kriav's molten core. He and his worker legions guided a great river of magma, creating tributaries and otherwise making it go where they wanted. Many of the volcanoes and magma wastelands of the super region Midrêth are the result of these super canals.
One legend says that Surtur controlled the lava flow, or at least planned it, from his fiery citadel Svaer Hodgrolf.