Experts say it’s impossible to tell from the returns how he generated such eye-popping losses, only adding to the mystery surrounding Trump’s taxes — and surely upping the pressure on him to finally release his full returns.

“The idea of one person, all by himself, having $1 billion in net operating losses? That’s something most tax lawyers and accountants don’t see in a lifetime,” said Alan Cole, an economist at the Tax Foundation.

Trump may have simply lost a ton of money, thanks to bad investments in casinos, hotels and the Trump Shuttle airline. It could be the result of him taking advantage of the particularly generous treatment the tax code affords the real estate industry. He could also be gaming the system, with aggressive moves to reduce his tax bill.

“As a 40-year tax guy, I looked at it and I thought, ‘Well, the numbers are interesting, but I have no idea what’s going on,’” said Dave Kautter, a tax lawyer at the accounting and consulting firm RSM.