It's not a shape, it's big and brown but what is it?

A few weeks ago at a friend's Isabella picked up a tree trunk that was part of a fairy playset and announced "look it's a barkindo"

After grilling her thoroughly we had not a clue what she was on about (It's not a shape was her only explanation) and put it down to her lastest trick of making up silly words.

Tonight in the car she was rambling on (seems par for course with three year olds) and mentioned barkindos. The conversation went like this

Me: What's a barkindo
I: It's like a barkindo
Me: What does it look like
I: Big
Me: What colour
I: Brown
Me: What does it do?
I: It goes boom
Me: Where have you seen them?
I: Far away
Me: In the sky?
I: No silly, it's not a planet
Me: Who has one?
I: Jessimae but not Alfie, he hasn't got a barkindo
Me: Can you sit on it?
I: No there are no chairs or sofas on a barkindo, jack frost can get to the top
Me: What is on a barkindo then?
I: Just a big hole at the top
Me: So what does a barkindo do?
I: It just goes Boom! Mummy
Me: Do you mean a volcano?
I: Yes a barkindo, it goes Boom!

And that has been bugging me for weeks! So obvious once you get it!