Can you believe your eyes? When it comes to these head-twisters, chances are you can't. Come around the globe with us on a tour of 'Did you just see what I just saw?'s.
1. Fata Morgana, Antarctica
Way up north (or way down south) the clear and pure air brings distant objects into sharp focus. Depth perception becomes impossible and the world takes on a strangely two-dimensional aspect. On maps and charts the early explorers meticulously laid down islands, headlands and mountain ranges that were never seen again. An amusing example of the phenomenon involves a Swedish explorer who was completing a description in his notebook of a craggy headland with two unusual symmetrical valley glaciers; he was actually looking at a walrus! Fata Morganas are caused by reflections off water, ice and snow, and when combined with temperature inversions, create the illusion of solid, well-defined features where there are none.
Touring the Antarctic? Look for a company that's a member of the International Association of Antarctic Tour Operators; it means they're into responsible tourism.
2. St Elmo's Fire, Edinburgh, Scotland
Herman Melville called it 'God's burning finger'. Caesar saw it on the javelins of his troops the night before battle. This spectacular effect (cause by the discharge of electricity from storm clouds to the earth) has always conjured thoughts of omens and divine intervention. It's often seen on the masts of ships during a storm; sailors would welcome the sight, as it usually comes at the point where a storm is quietening down. St Elmo was a protector of sailors, and it was seen as his calling card. The effect is frequently seen on the heights of Edinburgh's Castle Rock.
You can't miss the Castle, louring over the town from its volcanic perch. The Royal Mile will take you there.
3. Polar lights, Alta, Norway
A space spectacular, the polar lights are a dazzling Arctic and Antarctic display, their colourful sheets of light transforming the endless winter nights into natural lava-lamps. The polar lights – aka aurora borealis and aurora australis – form when solar particles, thrown out by explosions on the sun, are drawn by the earth's magnetic field towards the north and south poles, colliding with atmospheric gases to emit photons, or light particles. What results are brilliant sheets of green, red, white, purple or blue light.
With a latitude of N 69°, the Norwegian town of Alta is renowned as an excellent base to see the lights.
4. Brocken Spectre, Goslar, GermanyImage by Keiichirou Shikano
For thousands of years anyone lucky enough to witness this extraordinary optical phenomenon probably thought they were in the presence of God or undergoing their own spiritual rebirth. That's because the spectator is confronted with an image of their shadow surrounded by a halo of light, usually around the head. The phenomenon mostly occurs near mountain peaks when the air is moist and the sun is low. The name owes its provenance to the Brocken, which at 1141m is the highest peak in the Harz Mountains straddling the German province of Saxony-Anhalt.
Berlin Linien Bus has a service to the Harz Mountain's gateway town of Goslar; the trip costs around €40.
5. Green Flash, St-Jean de Luz, France
A favourite of those with romantic imaginations, the Green Flash (or Green Ray) seems to capture something of the ineffable and transitory nature of existence. It's an effect seen at the end of the sunset, when a green spot or a green ray seems to shoot out of the sun. The causes of the illusion are complex and have to do with the refraction of light, the thickness of the atmosphere and the curvature of the earth. Try for a glimpse of it in St-Jean de Luz, the town featured in Éric Rohmer's moody film Le Rayon Vert.
St-Jean de Luz is on the Basque coast, south of Biarritz; try some of its famous seafood while you're waiting for sunset.
6. New-growth conifers, Mt St Helens, Washington, USA
This is one for a spring day. As you approach Mt St Helens, in Washington state, you'll see that the new, pale-green growth sprouting on the dark-green conifers forms an eye-bending pattern, almost like an op-art painting. Spread across the scale of the forest, the effect is startling. Mt St Helens erupted spectacularly in 1980, causing its north face to collapse in a shower of rocks and releasing a massive ash cloud. Fifty-seven people were killed and the landscape was instantly turned into a featureless moonscape. Since then, the area has been protected and allowed to regenerate.
Visit the Mt St Helens National Volcanic Monument website to check out the webcam and visitor details.
7. Magnetic Hill, Ladakh, India
A land of high snowy passes and ancient gompas on the borderlands of Tibet, Ladakh is the kind of place where your imagination can run away with you. Here you can encounter the phenomenon of a magnetic hill, also known as a gravity hill, where vehicles left out of gear appear to roll uphill. This astounding effect has led to stories about how the magnetic force pulls planes off course. But in fact this is just a powerful illusion - the slope is actually slightly downhill, but the shape of the surrounding landscape and mountainous horizon mean that our usual reference points are obscured.
The hill can be found 30km from the historic capital of Leh along the Leh-Kargil-Baltic highway and is marked by a large sign.
8. Desert mirage, Nullarbor, Australia
This is a commonly observed phenomenon – a heat haze that makes the air shimmer and can make roads look wet. It's a mean trick, really. For exhausted travellers in brutal heat, the appearance of an illusory lake in the distance cruelly raises and then dashes hopes. On the other hand, if you're cruising comfortably along in a car with a mineral water to hand, the hazy refractions of light just add to the atmosphere of your road trip. Australia's Nullarbor Desert (its name means 'no trees') is the ultimate flat horizon. Driving along this seemingly endless road affords great opportunities for flirting with mirages.
Before attempting a Nullarbor crossing, make sure your vehicle is in excellent condition and you have plenty of water.
9. Paasselkä Devils, Lake Paasselkä, Finland
In England they're called will-o'-the-wisps or jack-o'-lanterns. In America they're called spook lights. The Scots call them spunkies. The phenomenon they're referring to is a light that appears at night, often in marshy ground. If followed it will back off ; it can also appear to follow you. Most cultures have seen such lights as evil spirits, luring travellers to doom, or harbingers of disaster. Finland's deep Lake Paasselkä is famous for mysterious balls of light; they've even been caught on film! In Finnish folklore, the lights are believed to mark the sites of treasure.
Traditionally, the Finns believed that early autumn was the best time to go looking for strange lights and the treasure below them.
10. Sun dogs, Timbuktu, Mali
A sun dog (parhelion when it's being posh) is an effect seen around the sun. It looks like bright spots of light (or 'mock suns') sitting on either side of the sun itself. It can last for hours. In earlier times it was seen as a frightening omen of bad times ahead. But when you know it's just innocent ice crystals making prisms in the air it's a lot less threatening. You'll have the best chance of seeing one when the horizon is flat. Timbuktu's baked-sand vistas and ancient mud temples could make a good setting for a sighting.