Follow the stairs down to the Furnace and take the S walkway to the W. Press against the wall, and walk N, crouching when necessary to avoid the crane that moves back and forth. Alternatively, you can destroy the crane, using two Stinger missiles, before proceeding across, though this will trigger the Alert phase. Head down the stairs, avoiding several guards and then exit through the NE door. Use the cargo elevator to the N to head down towards Metal Gear's Underground Maintenance Base. As the elevator moves, three guards will jump down and ambush you.