Karsten Erstad is a biologist from Norway.

But despite his experience of the animal world, he couldn't believe his eyes when, whilst skiing, he came across thousands upon thousands of earthworms.

At first, he thought they had been driven up from the ground - but there was 50cm of snow on the ground - the worms would surely have frozen.

Instead, it appears they rained down from the sky.

Karsten reported up to 20 worms per square metre, telling news outlet The Local: "I saw thousands of earthworms on the surface of the snow.

"When I found them on the snow they seemed to be dead, but when I put them in my hand I found that they were alive."

And he isn't alone. The finding made national news in Norway and prompted an influx of similar reports.

So how does it rain worms? And why? Well, no one is entirely sure. But it's thought the worms, starting to emerge from the ground towards the end of winter, were swept up by the wind. In the mountainous Norwegian coastline they quickly caught thermals - rising pockets of warm air more commonly used by birds of prey to circle - and were carried up into the sky.

Eventually, and after travelling some distance, they fall back to Earth as earthworm rain.

The phenomenon is rare - though it isn't new. In April 2011 a PE lesson at a school in Galashiels, south of Edinburgh, had to be cancelled after worms began raining onto the pitch.

Falling worms might be a bit unpleasant, especially if you aren't a fan of creepy crawlies - but they have nothing on these:

All the way back in 1894 it's believed Bath experienced a shower of jellyfish. Which sounds pretty terrifying, except no one's actually sure what they were. Some say jellyfish, others say tadpoles. Which is slightly less like something from a horror film.

On creatures from the deep, in August 2004, the village of Knighton, in Powys, was reported to have endured a rain of fish.

Similarly, in June 2009, reports of a rain of frogs and tadpoles came in throughout Ishikawa prefecture, Japan. Reports continued throughout the month.

But tadpoles, fish and even jellies pale in comparison to one of natures most terrifying swarms. And this one was caught on video.

An enormous colony of spiders was blown in to the town of Santo Antonio da Platina, Brazil, in February 2013. Whilst at first confused (and probably rather terrified) residents thought it was raining spiders, but in actual fact their enormous web had been picked up by the wind and dumped on top of the town, hanging from rooftops and power lines.

The phenomenon is apparently not unusual, and the skies around Sao Paulo and neighbouring areas are often full of a species of spiders known as anelosimus eximius. They aren't big - just half a centimetre or so across. But they make up for that by living in colonies of up to 50,000, and ganging up to take on much larger prey than most spiders can handle.

In fact, their vast communal webs can cover entire trees or buildings. Don't have nightmares.

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