Every summer, my parents, my six-year-old brother and I go to stay in a cabin by a lake called Vidöstern in Tånnö in southern Sweden, not far from where we live. I like to build sandcastles on the beach, or find rocks to skim across the water and see how many times I can make them bounce. Mamma says she used to play and swim in the lake when she was little, too.

On 15 July this year, I was playing on the beach with my friend, when Daddy told me to get a buoy from the cabin: he said the water level in the lake was very shallow and we had to warn any boats that might come along because it was dangerous. He said it had been the hottest summer for 260 years.

I waded into the water and it was very soft on my skin and refreshing, a little bit cool but not too cold. It was a nice feeling because the sun was shining and I was very hot. Daddy was begging me to rush so he could watch the World Cup final, but I like to take my time about things so I ignored him.

I was crawling along the bottom of the lake on my arms and knees, looking for stones to skim, when my hand and knee felt something long and hard buried in the clay and sand. I pulled it out and saw that it was different from the sticks or rocks I usually find. One end had a point, and the other had a handle, so I pointed it up to the sky, put my other hand on my hip and called out, “Daddy, I’ve found a sword!”

I felt like a warrior, but Daddy said I looked like Pippi Longstocking. The sword felt rough and hard, and I got some sticky, icky brown rust on my hands. It started to bend and Daddy splashed up to me, and said I should let him hold it. It was my sword and now he was taking it away! I gave it to him in the end.

I ran to my mamma and my mormor – my grandma – and some other relatives who were all sitting outside having fika, which is Swedish for having a sit-down with coffee and cookies. I was yelling, “I found a sword, I found a sword!” Daddy went to show it to our neighbours, whose family has lived in the village for more than 100 years, and they said it looked like a Viking sword. Daddy didn’t get to watch the football in the end.

The sword found by Saga Vanecek. Photograph: Jonkopings museum

When he showed it to an archaeologist, she said she had goosebumps and that it was at least 1,000 years old. Actually, they now think it’s 1,500 years old – from before the Vikings. She called it “sensational” and said nothing like this had ever been found in Scandinavia before, and that maybe I had found it because of the low water levels. She made me promise not to tell anyone because she and other archaeologists wanted to see if there was anything else buried in the lake; they didn’t want anyone else to come and take the treasures.

It wasn’t hard to keep the secret. But I did tell one of my best friends, Emmy, and now I know I can trust her because she didn’t tell anybody, except her parents – but they promised not to tell anybody else, so that’s OK.

This month, the archaeologists finally came to search the rest of the lake and they found a brooch that is as old as my sword, and a coin from the 18th century. Then they announced the news and I could finally tell everyone at school. I came back from gym class and the whiteboard said, “Saga’s sword” and there were balloons, and the whole class got to have ice-cream.

I had to give the sword to the local museum – Daddy explained that it’s part of history and important to share it with others. I felt “boo” that it’s gone away, but “yay” that other people will get to see it. I’m going to try to raise some money to make a replica sword that I can keep.

People on the internet are saying I am the queen of Sweden, because in the legend of King Arthur, he was given a sword by a lady in a lake, and that meant he would become king. I am not a lady – I’m only eight – but it’s true I found a sword in the lake. I wouldn’t mind being queen for a day, but when I grow up I want to be a vet. Or an actor in Paris.

Now, whenever I go swimming in the lake, I will be looking to see what I can find. It feels like that lake might be a little bit magic. On that day I felt a little bit magic, too.

• As told to Moya Sarner

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