The long-concealed market value of Tokyo’s largest skyscrapers is being unveiled by activist investors.

In Japan, there’s a huge gap — ¥22 trillion ($143 billion) by one estimate — between how companies value their real estate assets on their books, versus what those same properties would fetch if sold in the current market. That comes from two factors: First, many of the country’s firms have held onto properties for decades, each year writing down the cost of fixed assets due to annual depreciation, a common accounting practice. But at the same time, property prices have soared.

The result is that billions in value can be unlocked by pressuring companies to sell off these holdings, a tactic that activist funds are now employing. This was illustrated last week when Japan developer Mitsui Fudosan announced it would aim to sell off ¥2 trillion in real estate assets over the next three years as part of a new business plan — just two months after news that New York-based activist hedge fund Elliott Management had built a stake in the company.