Sea turtles have been travelling the world's oceans for millions of years. Yet their survival depends on a surprisingly narrow moment in their lives: the journey from sea to sand, and later from sand back to sea.

Northern KwaZulu-Natal is particularly important because its beaches support nesting by two very different species. Understanding those differences makes the night more meaningful than a simple wildlife checklist.

The loggerhead (Caretta caretta) gets its name from its large head and powerful jaws. Those jaws help it feed on hard-shelled animals, including crabs and molluscs.

Loggerheads migrate across large areas of ocean, but nesting females must eventually return to a suitable beach. Once ashore, a female excavates a nest in the sand, deposits her eggs and covers them before returning to the sea.

The process is physically demanding. On land, a turtle that is well adapted to life in the ocean becomes vulnerable to disturbance, obstacles and human interference.

The leatherback (Dermochelys coriacea) is the largest living sea turtle. Rather than the familiar hard, plated shell of other sea turtles, it has a flexible, leathery covering over a framework of small bones.

It is also a long-distance ocean traveller, with a diet dominated by soft-bodied animals such as jellyfish. That diet links its survival to healthy marine food webs and makes plastic pollution a particular concern: floating plastic can be mistaken for prey.

Seeing a leatherback on a nesting beach, if you're fortunate enough to encounter one, puts the scale of this animal into perspective. An animal adapted to open-ocean life has hauled itself onto land to reproduce.