At the base, they find large areas of the underground mall still standing as they were the morning of September 11, nearly untouched.

CNN: workers

Since tax evasion is a major driver of the underground economy, efforts to expand the tax base and improve tax collection can help curtail illicit flows, says Kar.

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The 25.5 acre (10.3 hectare) Neatishead base, near Wroxham, includes a high-security fence, a Cold War underground bunker and a helipad.

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