To safely tap this for your Magma Forges and Smelters, go down one Z-level and dig a tunnel a few squares up to it and smooth and carve a fortification in the end of it for magma to flow through. Seal the tunnel off from your fortress with a wall or floodgate, then go back up one Z-level and channel from the magma pipe to your tunnel's fortified end. Beware of magma men and fire imps during the last phase!