Sea turtles are ancient mariners.

Present in all but Earth’s coldest oceans, these marine reptiles are well-adapted to a life on the move and have been around for millions of years.

FACT 1: Sea turtles have existed since the time of the dinosaurs.

The earliest known marine turtle lived about 120 million years ago.

Named Desmatochelys padillai, it was six feet long and had the characteristic features of modern sea turtles – including a carapace, or top shell, and paddle-like limbs.

During the Cretaceous period, about 145.5 to 66 million years ago, there were dozens of species of sea turtles. Today there are seven species.

The largest is the leatherback. Growing as long as 8 feet and weighing up to 2000 pounds, it is larger than known fossils of its prehistoric ancestor Desmatochelys.

FACT 2: Sea turtles can lay more than 150 eggs at a time.

During nesting season, female turtles migrate to beaches and dig nests up to two feet deep. They deposit up to 150 eggs in one installment, called a “clutch.”

Once a female lays her eggs, she leaves the nest and returns to the sea. After incubating for about two months, the hatchlings emerge and scurry towards the ocean.

Fewer than .1% of hatchlings survive to adulthood, in part, due to numerous predators on the beach. Emerging in large numbers increases the chance that some will survive.

FACT 3: A sea turtle’s sex is determined by temperature.

Unlike most vertebrates, a sea turtle’s sex is not determined by sex chromosomes. Instead, the temperature of a nest determines the sex of the hatchlings.

When temperatures are warm (at about 88 degrees F), most of the hatchlings will be female; but when temperatures are cooler (less than around 82 degrees F), most of the hatchlings will be male.

The heat generated within the nest also influences the sex of hatchings. Eggs in the center are warmer and more likely to become females than those on the edge of the nest.

FACT 4: Sea turtles use Earth’s magnetic fields to navigate.

Sea turtles have geomagnetic abilities, which serve as an internal GPS.

The turtles have particles of magnetite, a magnetic mineral, in their brains. The magnetite likely plays a role in orienting sea turtles to Earth’s magnetic poles, similar to a compass.

This “magnetic memory” helps sea turtles migrate between food sources and mating and nesting areas. Some navigate back to the beaches where they hatched, when it comes time to lay their own eggs.

Fact 5: All seven sea turtle species are threatened with extinction.

Once abundant, sea turtle populations have dramatically declined in the past two centuries.

Fishing is a major threat to sea turtles, as they become caught in fishing nets as “bycatch,” or accidental catch. Illegal harvesting of turtle eggs, trafficking in turtle products, and ocean pollution also are concerns.

Policies have been enacted to reduce bycatch, protect turtle habitats, and prohibit the killing of sea turtles and their eggs^.

By continuing this kind of intervention, humans can help stop the decline of sea turtles, helping keep these ancient mariners in the world’s oceans.