Biology
Absence of Sunlight in the Deep Ocean
Quick fact
Sunlight can only reach about 200 meters below the ocean surface; beyond that, the water is in perpetual darkness, and yet it is the largest habitat on Earth by volume.
Why this is interesting
You've probably swum in the ocean and felt the sun's warmth. But what happens when you dive deep enough that the sunlight disappears? The deep ocean is a world without light, yet teeming with life.