All the walkthroughs I've read say to access this part of the map via an underground route once the final boss battle has been initiated, but when I try to go this way, I run into a grenade hatch (eg it's glowing green) at the top of a duct, and I get a grenade to detonate in mid-air to get past it. This obstacle occurs below and to the left of the right-most save room. Am I just doing it wrong? Is there an alternate route I'm overlooking? Help! I need to get both the passkeys at the top right part of the map!