Just finished the new Area 15, it's pretty good, has fun secret objectives, but they're hidden well.
>restore power to the teleporter with terminals with a big red button: one in control room, one in dig site 2, one in lab 2 and one just behind the teleporter
>find a keycard hidden in the control room or one of the labs
>use keycard to open the basement available through an elevator in the control room
>restore power in the basement, get a vial from the newly opened room and fill it with souls using a device similar to one found in Middle School of the Dead
>use the vial on the secret weapon, the riftmaker, pick it up, create rifts and jump on them until you get to a secret place to get an artefact
>use the artifact on the golden alien bowl
>find 4 CDs, one in the basement, one in the control room, one in or near the teleporter, and one in lab 2
>copy data on the CDs from 4 terminals
>find a satellite dish with a blinking light out of bounds (you should see it from outside the entrace to the teleporter, looking upwards, over dig site 1) and shoot it down with the riftmaker
>pick it up and bring everything to the crashed chopper, send the data
>go back to the basement, fight a nightmare berseker, and pick up his essence
>bring it back to the golden sphere and feed it with souls
>accompany the cristal that pops up to the teleporter
>countdown starts, find the escape chopper on the balcony near the entrance to the secret room where the artefact you got using the riftmaker was
Should be done in 20-25 waves. 8/10 very fun map overhaul.