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In dark heart, when you past the first check point in the tunnels, turn the corner & the light is showing an extremely large shadow of a bug. You know it's just a small bug, but there is always some doubt that maybe it's a gigantic super bug. Just thought this was a smart little insert.
The cinematic scene during the evacuation where a man is trying to get his wife out from under a block, and he argues with the marines as you watch the building collapse & kill the man & women as the hummer speeds off leaving them. That was an intense & highly emotional moment in a game that is full of tense moments.
When the queensboro bridge is falling down & you have to hit "x" to grab a ledge. You hold on, and think you'll climb up as people fall to their death. Ends up a taxi slides off & knocks you into the river. I actually died in my first play through because I missed the grab & fell in the water. Never would think you would end up in the water anyway.
The parts of the story where you are going for an objective, and the massive flood wipes it out, and you end up on some wild tangent. It happens once as you race to evacuate before the flood, but get wiped out after they blow the dam. It happens again when you are in the glass wall building & the glass cracks flooding the building, and making your objective shift yet again.
Also, it semper fie or die if you hit the tactical options, you slide through a pipe, and flank the enemy with a HMG. On several levels hitting the tactical options make for a fast paced jolt to a position that is highly advantageous, and the resulting gameplay is epic as well as player generated, even if the tactical assessments hint at what you are supposed to do. I suggest a re-play through to find the levels that have fast paced action routes mapped out in the tac assessments & giving them a run-through.
In Dark Heart you can grab an alien at the end near the sinkhole & jump in it, it kills the alien & puts you at a spot part way back on the level without killing you, just thought that was interesting.