Why are these men wearing all that gear and kissing their loved ones goodbye just because they're going into that tiny, little pit in the ground? Because that is the opening to the world's deepest cave.

That shallow-looking opening is the tip of an iceberg that is larger than many of earth's tallest manmade structures put together. Krubera Cave stretches more than 7,208 feet (2197m) toward the earth's core.

"This will be the last time we see light for a long, long time."

Located in the country of Georgia, this is the only cave on earth deeper than 2,000 meters, and it takes 27 days to reach the bottom.

Imagine the many Empire State Buildings-worths of distance you might descend after having no more natural light to guide your path.

Creepily enough, the pitch black depths of Krubera Cave are home to an ecosystem of deep-dwelling insects that never in their lives see or feel sunlight.

In the veins of the earth.