Year 6 KS2 SATs — Reading Prep (Set 2)

More than a kilometre beneath the surface of the ocean, sunlight never reaches. Here, in a place scientists call the midnight zone, the water is a permanent, crushing black — and yet it is far from empty.

Many of the creatures that live here have solved the darkness in the same surprising way: they make their own light. This ability, known as bioluminescence, comes from a chemical reaction inside an animal's body, similar to the glow of a firefly on a summer evening.

The anglerfish is perhaps the strangest example. A thin rod grows from between its eyes, tipped with a pouch of glowing bacteria that dangles like a lantern. Smaller fish, drawn to the light out of curiosity, swim close enough for the anglerfish's enormous jaws to snap shut around them.

Not every creature uses light to hunt, however. Some squid release a cloud of glowing ink instead of the usual dark cloud, startling a predator just long enough to escape. Others flash light along their bodies purely to communicate, blinking messages to one another the way a lighthouse might signal to a ship.

Scientists believe as many as nine out of every ten deep-sea species may be bioluminescent in some way — meaning that far from being empty and lifeless, the deep ocean might be one of the most brightly lit places on Earth.