In a World War II bunker east of Frankfurt, a steel door weighing over four tonnes protects Germany's largest reserve of rare earths, a treasure at the heart of rising geopolitical tensions.

The exact location is confidential and the site is under close video surveillance.

This is where Tradium, a German company specialised in trading rare earths, keeps thousands of barrels of the precious materials -- almost all from China, the world's biggest producer.

The materials in the bunker -- such as dysprosium, terbium and neodymium -- are essential for the manufacture of crucial modern technology including smartphones, electric cars and wind turbines.

Tradium, which employs fewer than 40 people, expects to reach a turnover of 300 million euros ($346 million) this year.

In the midst of the U

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