Make your way north to where you found Indy and take the northwest exit again. Cross the bridge and make your way back to the large area and this time, take the northeast exit (the northwest exit takes you back to where you fought Croix). Continue north and go east at the fork (going west will take you to the Sun Ruins, which you can't enter unless you obtain the lithograph fragments). Enter the doors along the northernmost wall and you'll find yourself in the Sealed Ruins.
