This is it - the last secret in Tomb Raider II. Instead of moving the box under the zip line, pull it out one more square. From the area of ledge to the left of the zip line, leap down to the flat area on the rocks below (to the left). Go down the flat area and turn right at the end. Step onto the square outcropping, then jump across the lava trench to the deep niche cut out of the rocks (just a shade to the right). Turn around and leap to the flat portion of rock back on the other side of the trench (a shade to the right). Pull up into the opening and follow the passage all the way to the end - the gold dragon is waiting.

To escape, retrace your steps to the lava trench room. From the opening to the passage, leap straight across the trench to grab the ledge. Pull up and backflip - you should land on safe ground. Turn left and take a look at the exit door. Go ahead. Jump on through. Let's finish this quest.