Four storeys underground, kids are swinging off monkey bars and digging in a sand pit.

Sausages are sizzling on a council barbecue and palm trees hang overhead.

This isn't the local park but a former World War II oil tank deep underneath Sydney.

It is like urban exploring, but for children.

Before: The Tank at the Art Gallery of New South Wales. . . After: How it looks today with Mike Hewson's installation. . .

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The Tank at the Art Gallery of New South Wales. / How it looks today with Mike Hewson's installation.

Oil tanks were constructed in Australia during WWII to protect fuel from aerial attacks.

And because of that, engineer-turned-artist Mike Hewson says this install

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