The Challenger Deep, the deepest known point of the ocean floor, lies in which trench?

The correct answer is C, the Agulhas current. It is a warm current of the Indian Ocean that flows southward along the eastern coast of southern Africa, carrying tropical water towards the Cape before most of it turns back into the Indian Ocean. Along with the Mozambique and West Australian currents it makes up the southern Indian Ocean gyre.

Option A, the Florida current, belongs to the Atlantic; it carries water out of the Gulf of Mexico past Florida and feeds the Gulf Stream. Option B, the Canary current, is also Atlantic, a cold current flowing south along the north-western coast of Africa past the Canary Islands. Option D, the Kurile or Oyashio current, is a cold current of the northern Pacific that flows south past the Kurile Islands towards Japan, where it meets the warm Kuroshio.

The correct answer is B, Peru. El Nino, Spanish for the child, is the warm surface water that appears off the coast of Peru and Ecuador around Christmas in some years, replacing the normally cold Peru or Humboldt current. When it develops strongly the trade winds weaken, the anchovy catch collapses because nutrient-rich cold water stops rising, and Indian monsoon rainfall tends to be below normal.

Option A, Colombia, faces both the Pacific and the Caribbean but lies too close to the equator and is not the coast the phenomenon is named for. Option C, Chile, is washed by the same cold Peru current further south, yet the warming is described and measured off Peru and Ecuador. Option D, Bolivia, is a landlocked country in South America with no sea coast at all, so no ocean current can run along it.

The correct answer is A, the Pacific. The Ring of Fire is a horseshoe-shaped belt running around the rim of the Pacific Ocean, from the Andes up the west coast of the Americas, across the Aleutians and down through Japan, the Philippines and Indonesia to New Zealand. It holds the majority of the world's active volcanoes and is the source of most large earthquakes, because it follows the lines along which oceanic plates are being subducted.

Option B, the Indian Ocean, does have the seismic Sunda arc on its eastern edge, but that arc is counted as part of the Pacific ring and the ocean as a whole has no such belt. Option C, the Atlantic, is widening rather than closing, so its activity is concentrated along the Mid-Atlantic Ridge and is mostly gentle. Option D, the Arctic, is tectonically quiet apart from the slow spreading ridge that continues from the Atlantic.

The correct answer is B, 97.2 percent. Textbooks describing the distribution of water on earth give about 97.2 per cent as ocean water, roughly 2 per cent locked in ice caps and glaciers, and well under 1 per cent as groundwater, lakes, rivers, soil moisture and water vapour together. This is why fresh water is described as a scarce resource even though the planet looks blue from space.

Option A, 94 per cent, option C, 90.2 per cent, and option D, 92.2 per cent, are all too low and would leave an impossibly large share of the planet's water as fresh water. The safe way to remember the set is that the oceans take a little over 97 per cent, the frozen ice sheets of Antarctica and Greenland take most of what is left, and everything that human beings actually use comes out of the small remainder.

The correct answer is A. Spring tides occur at new moon and at full moon, when the sun, the earth and the moon lie in a straight line, a position called syzygy. The gravitational pulls of the sun and the moon then act along the same line and reinforce each other, so the high tides are unusually high and the low tides unusually low. Spring tides therefore come twice in a lunar month.

Option B describes the position at the first and third quarter, called quadrature, when the two pulls partly cancel and produce the weakest tides of the month, the neap tides. Option C, the moon at apogee, makes tides slightly weaker rather than stronger, since the moon's pull falls off with distance. Option D is the common trap: the word spring here means to leap up, and the tide has nothing to do with the spring season.