1. Dirty thunderstorms, aka volcanic lightning, occur when lightning is produced in a volcanic plume.
They look like the entrance to Hell!
2. Oh wait, here's The Door to Hell, a gas fire in Turkmenistan accidentally ignited by scientists in 1971 and still burning. Oops.
3. Flammable ice bubbles: frozen bubbles of methane, trapped beneath Alberta's Lake Abraham.
4. The Catumbo Lightning, which occurs during 140 to 160 nights a year, 10 hours per night and up to 280 times per hour.
5. Christmas Island's Red Crabs: Each year an estimated 43 million land crabs migrate to lay their eggs in the ocean.
Authorities close most of the island's roads during the migration, which normally takes at least a week.
6. Monarch butterflies: The eastern North American population is notable for its southward late summer/autumn migration from the USA and Canada to Mexico, covering thousands of kilometers.
No individual butterfly lives through the whole migration. Female monarchs lay eggs and their offspring continue the migrations.
7. Surreal spiderwebs: Fleeing torrential floodwaters near Wagga Wagga, Australia, thousands of spiders cover fields with cobwebs.
8. Namibia's mysterious Fairy Circles: Studies suggest that a sand termite is responsible for their creation.
9. Underwater crop circles in the ocean off Japan: created by a male pufferfish in order to woo females.
10. Spherical boulders in New Zealand: exhumed from the mudstone enclosing them by coastal erosion.
11. The Great Blue Hole: a large submarine sinkhole off the coast of Belize, over 300m across and 124m deep.
12. The Black Sun: Huge flocks of up to 50,000 starlings form in areas of the UK just before sundown during mid-winter. They are known as murmurations.
13. The Sardine Run: occurs from May through July when billions of sardines move north along the east coast of South Africa. Their sheer numbers create a feeding frenzy along the coastline.
14. The Giant's Causeway, Northern Ireland: an area of about 40,000 interlocking basalt columns, the result of an ancient volcanic eruption.
15. Sailing stones in Death Valley, USA: a geological phenomenon where rocks move and inscribe long tracks along a smooth valley floor without human or animal intervention.
16. Tidal bores on the Amazon in Brazil and the Severn in England: a tidal phenomenon in which the leading edge of the incoming tide forms a wave of water that travels up a river against the flow.
17. The Flowering Desert: occurs in the Atacama Desert, Chile, in years when rainfall is unusually high. Normally the region receives less than 12mm of rain annually.
18. Circumhorizontal arcs, misleadingly known as fire rainbows: an optical phenomenon featuring an ice halo formed by plate-shaped ice crystals in high level cirrus clouds.
19. Lenticular clouds over Mount Olympus: stationary lens-shaped clouds that form in the troposphere. Because of their shape, they have been offered as an explanation for some UFO sightings.
20. Mammatus clouds, aka "mammary clouds" or "breast clouds": a meteorological term applied to a rare pattern of pouches hanging underneath the base of a cloud.
21. Polar stratospheric clouds: also known as nacreous clouds (from nacre, or mother of pearl, due to their iridescence).
22. Undulatus asperatus aka "roughened or agitated waves": This cloud formation has been proposed as a separate cloud classification by the founder of the Cloud Appreciation Society and would be the first new type of cloud recognised since 1951.
23. Tanzania's Lake Natron: a salt lake fed by mineral-rich hot springs that is the only regular breeding area in East Africa for the 2.5 million lesser flamingoes.
The flamingo population has been adversely affected in recent years by suspected heavy metal poisoning, and the lake is currently under threat by a proposed soda ash plant by Tata Chemicals.
24. Canada's saline endorheic alkali Spotted Lake: contains some of the highest quantities of magnesium sulfate, calcium and sodium sulphates in the world.
25. Bioluminescent waves on a beach in the Maldives: Various species of phytoplankton are known to bioluminesce; when washed ashore by the tides, their chemical energy is turned into light energy.
26. Bioluminescent dinoflagellates + the right conditions = Red Tide: a condition where the dinoflagellates become so numerous that the water takes on a muddy reddish colour.
27. Light pillars: an optical phenomenon formed by the reflection of sunlight or moonlight by ice crystals that are present in the Earth's atmosphere.
The light pillar looks like a thin column that extends vertically above the source of light. They have been known to produce false UFO reports.
28. Rainbow eucalyptus aka rainbow gum: patches of outer bark are shed annually at different times, darkening and maturing to give blue, purple, orange and then maroon tones.
29. Frost flowers: ice crystals commonly found growing on young sea ice and thin lake ice in extremely cold, calm conditions nearing -22C or -7.6F.
30. Snow chimneys on Mount Erebus, Antarctica: the southernmost active volcano on Earth.
Special mention: The Moskstraumen is a tidal whirlpool, one of the strongest in the world, that forms in the Norwegian Sea*.
The Moskstraum was the inspiration for Edgar Allan Poe's short story A Descent into the Maelström (1841), which brought the term maelstrom into the English language.
An earlier version of this post did not properly cite the source of information relating to Monarch butterfly migrations.