Entering the crypt to obtain the green key card.
Gear: Bear Suit, Night vision or torches, Weapons, and a lot of ammo

1. Bear Suit - You will need a 4 bear pelts and a ghillie suit. You can craft a ghillie with 10 burlap straps and 4 netting, make sure you craft the SUIT nothing else. You need to kill 1 bear to get pelts. You will need a leather sewing kit once you killed the bear, once you find an skin a bear simply combine the pelt with the leather sewing kits in order to create 4 processed bear leather (this does not use up the Leather Kit). Combine the 4 processed pelts with the ghillie suit in order to craft the bear suit, you will need this for the next stages. Don't put on the bear suit yet because it will increase your temperature drastically.
2. Ice Sheet - Make your way to the ice sheet, top right part of the map. A boat is a wonderful way to explore the island. Once you get to the island you will need to follow a stream all the way until you hit several ancient buildings. Once you hit the end of the river, park your boat and proceed to the Ice Temple by following the path. Watch out for Zombies.
3. Ice Temple/Green Key card - The ice temple is a maze full of dangerous. Here you need to find a switch that spawns at 12 potential spawn points. You will need to find it and trigger it, then you need to find the hidden door. Make sure you study the maze before you flip the switch because the hidden door is on a timer once the switch is flipped. Once you reach the end, the switch triggered an event. It is recommended to wait an extra 5 minutes once the event ends to proceed forward to collect the final key card.
4. The Carrier - Once you have the green key card, head to the carrier which is located at the North west side of the ice sheet. The island is full of bears and dangers so be careful as you travel. Recommended by boat or helicopter.
5. Look for a door on the carrier and use the green key card.