Secret government files on possible spaceship sightings have been shown for the first time.
They give details of people's reports of glimpsing aliens, seeing UFOs - unidentified flying objects - and watching odd lights in the sky.
So we wondered what weird things you've seen. Do you think you've ever seen a UFO or a ghost? Or maybe a strange-looking animal?
Or maybe you've seen a weird TV programme or film?
This topic has now closed. Check back on the Chat page for other things to talk about.
"I once was screamed in the ear at on a criuse ship. I looked around but nothing was there!"
Ella, 10, Penarth, Wales
"Me and my friend Suzie were at a sleepover and there was a huge greenish-black 20 foot tall thing moving across a field we could see from the window. We went to the field the next morning and the grass was all crumpled!! EEE!!"
Alice, 13, Cumbria, England
"I once saw a strange light in a field next to my house and it wasn't a tractor because there wasn't a noise. When I got my mum to show it to her, it had gone."
Emily, 11, Essex, England
"In January 2008 my mum got a text from her friend saying to look at the sky. We saw it straight away - 3 glowing lights in a row. I thought they were aliens. My mum and dad said there's no such thing as aliens. But I still think they were. And the glowing lights were in the same sort of shape as a UFO!"
Sophie, 10, Bristol, England
"I saw a ghost in the corner of my bedroom!!"
Daniel, 12, Indiana, USA
"I've seen a white figure behind my friend and I got really scared so I screamed and ran."
Salma 12, Bradford, England
"I think there is no such thing as unidentified flying objects. And if there was why would they be down here? Why not up in the sky? And if there was any proof of them, then I would be convinced."
Jordan, 13, Wolverhampton, England
"I have seen a ghost on my friend's camera when she was taking a picture of her little boy!"
Elisha, 14, Wiltshire, England
"I have seen some very weird lights in the sky, so I reported them to our local paper, they said someone else had too!?"
Jonathan, 11, Leicester, England
"Me and my brother saw a UFO when I was about 9 years old. We were both out of bed because we could not get any sleep and so I was looking out of my bedroom window when I saw a glowing object in the sky. I shouted to my brother to come and see what I saw. It was very fast moving but I saw a little bit of it and so did my brother."
Jazmine, 14, Essex, England
"One night I went for a walk at the nearby park in Kesgrave and I saw what can only be described as a green 2D hexagon circling in the sky. But when I told my parents they didn't believe me."
Bustopher, 12, Suffolk, England
"When I was in year 5 me and my friends were doing a science experiment to look at stars. And then a triangular thing with lots of light appeared zooming past! We were so scared!"
Smelly Melly, 11, Birmingham, England
"When I was at my friend Patrick's house we saw green lights in the shape of a man."
Gabriel, 11, Kent, England
"I saw a strange UFO floating across the sky. It looked like a light but I really think it's a UFO because something jumped out of it like an apple or something!"
Nicole, 9, Stockport, England
"Once in literacy I was writing with a pen, I put it down on the desk next to me to read something and then I went to pick it up and it was nowhere to be seen! I looked under the desk and under all the others - it was not there."
Abby, 10, Huddersfield, England
"I'm sure I've seen flashing lights in the sky and they are not planes!"
Christia, 12, Manchester, England
"I have seen flashing rings of light outside my room before, they were going pink, blue, green, pink, blue, green. It was so scary."
Kelsie, 12, Cambridgeshire, England
"I have been seeing weird flashes outside my window. You might be thinking it's car headlights, but I'm pretty sure that aliens exist. Very WEIRD."
Najing, 8, London, England
"Once when me and my mates Hannah and Rebecca went to the graveyard [we were just having a look]. We saw this black silhouette of a man who was just standing there and we both screamed and he was gone. We ran to the spot but no-one was there. Scary. :S"
Helen, 13, Buckinghamshire, England
"I've seen a weird bright light on the horizon before. I was in the car and suddenly a light flashed on the horizon, but it was really big and bright."
Emily, 11, Essex, England
"I was once riding my bike, and I saw something shiny in the sky. It was gone soon after I saw it. I look for aliens, spaceships and ghosts through my telescope!"
Wassib, 10, Leeds, England
"I saw a red frog with black spots in a bush."
Livvy, 10, Derby, England
"I saw a guy dressed as a banana walking down the street :)"
Rebecca, 13, Glasgow, Scotland
"Me and my auntie were playing in a field by her house, we ran into the middle of the field and we saw 2 big circles cut into the grass. We were walking round them and we saw little tufts of grass in different shapes! We got scared after a while and went home!"
Demelza, 12, Cornwall, England
"I have seen my sister being born."
Tapiwa, 10, West Midlands, England
"In year 6 we went to Devon and my teacher's toenail fell off. It was disgusting!:-P"
Helena, 12, London, England
"Once I tried to pick a stick but it kept pulling down like it was magnetic but it wasn't I don't think."
Cosmo, 11 ,Cambridge, England
"I found a piece of metal with strange writing on it. I looked on the internet about how to write different languages, but still this writing was unidentified. So I think is some sort of alien ship metal."
Maeve, 11, Cornwall, England
"The weirdest thing I've ever seen is when I was at school and a boy in my class can pop his eyes out. It looks pretty weird!"
Honour, 12, Suffolk, England
"I saw a UFO flying above my head on the way home from school a few days ago but none of my friends believe me."
Reuben, 11, Kent, England
"I've seen a skull of someone thousands of years ago, it was very freaky and it was being preserved."
Claire, 11, Co. Tyrone, Northern Ireland
"I was sitting in my room listening to music and everyone else was downstairs. My light turned off and so did my music. The lights in the hallway were still on and so were all the others - something freaky was going on."
Sazy, 12, Eastbourne, England
"I went to Scotland with my family and I saw the Loch Ness Monster! I'm sure it was. It was black coloured!"
Beth, 14, Merseyside, England
"The weirdest thing I have ever seen is a dog riding a bike. Weird or what?!!!"
Chelsea, 10, Merseyside, England
"I always sleep with my curtains open, but once, when I woke up in the morning, my curtains were drawn. I never draw them, and it freaked me out."
Alice, 13, Essex, England
"Once I saw a big, white, fluffy dog. I didn't know it was a dog and thought it was a big polar bear!"
Isabelle, 10, Surrey, England
"At my gran's house I saw this mysterious figure on the railway, when I blinked he was gone!!"
Alana, 11, Manchester, England
"I saw a two-headed caterpillar in the valley. It was making strange shapes and real letters on my hand! CREEPY!"
Caitlin, 9, Barnsley, England
"A few days ago my phone was out of charge and it was on top of my bookcase. Suddenly I heard it turn on, so I went into the back room and had a look, but it was off. I tried to turn it on and it worked! Freaky."
Kira, 11, Coventry, England
"I have seen ET on the television. I think he is real."
Josephine, 7, Chelles, France
"I once saw a ghost and I thought it was friends playing a prank on me."
Andile, 12, Kent, England
"I saw a creepy looking monster in the bin."
Olly, 12, London, England
"I have seen my dad in a bikini!"
Ashleigh, 13, Norfolk, England
"I've seen a camel roller-skating in Tunisia."
Matthew, 11, Southampton, England
"I was in my room and my DVD player turned itself on. The remote wasn't anywhere near me! It really freaked me out!"
Sophie, 14, Kent, England
"I have seen a very weird thing in the sky. It looked like a flying saucer, had loads of lights and freaky voices... possibly aliens."
Aaliyah, 12, Huddersfield, England
"I saw some weird looking creature in my garden that had two heads. I don't know if it was a dream or not, so I'm not sure!!"
Amelia, 13, Powys, Wales
"I was eating my dinner yesterday and just as I was putting a tomato in my mouth I noticed it was engraved with the Arabic way of writing God."
Sanna, 12, Huddersfield, England
"About three years ago I was in my bedroom looking out of the window when I saw a mysterious misty shape moving through the valley where I live. When I looked closer I saw details such as a funnel and a number plate on the side - it was a train... a ghost train. When I asked my dad, he said that a train track used to run through the village about 100 years ago and the number of the train that used to pass through most often was the 465 - that was the exact same number that I had seen on the side of the ghost train."
Olivia, 11, Lancashire, England