Since I saw that video of someone filming the island from a boat and the pictures of that blue tent below the weird temple, I convinced myself that there could be underground shit there.
I found Apple's Maps has very good imagery of the island. The island is even 3D "modeled" so you can get a better sense of heights.
The island
At first I suspected that small white thing beside the bigger white "tent".It kinda looks like an entrance.
Suspicious white tent
Then I looked around and BAM! Look at this!
Underground 1
Underground 2
Undeniably there are underground spaces on that island.
What is that underground space for?
What size is it?
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mattLessner ago
If you tilt your screen so the right side is down and left is up, in both of those undergound pictures the undergound entrace door resembles a face. Unusual that both angles would appear that way; http://picgur.org/image/F0n http://picgur.org/image/F0v