Pedestrians walk past Trump Tower on Wednesday, Aug. 26, 2015. (Michael Nagle/Bloomberg)

Donald Trump files taxes by the foot. He posted a photo of himself signing his return last fall, and the thing is at least 3 feet high, individual bound chunks of paper spelling out in detail the ebbs and flows of Trump's personal fortune.

Signing my tax return....

A post shared by Donald J. Trump (@realdonaldtrump) on

In recent weeks, Trump has come under increased pressure to release those returns, particularly after Mitt Romney suggested that the returns might include "a bombshell." Marco Rubio and Ted Cruz each released the top sheets from their own filings, hoping to add to the pressure on the front-runner. Trump insists that he can't and won't release them, though -- can't because he's under audit, and won't because the returns wouldn't show anything interesting anyway.

The former point is valid, as we learned when we spoke with tax attorney Steven Goldburd, partner at Goldburd McCone. An audit is an investigation, and a lawyer would recommend that a client not turn evidence over to the media. (Though, as Romney pointed out, Trump could release past returns.)

The latter point, though -- about not learning much from the return? That's not necessarily the case.

Trump works mostly in real estate. The disclosure he filed with the FEC include page after page of financial interests, many of which are limited liability corporations that obviously relate to real estate investments.

A section of Donald Trump's FEC filing. (FEC)

As it turns out, those properties could be the key to paying very little in federal income taxes -- or nothing at all.

We went back to Goldburd to explain how or why someone might be able to avoid paying federal income taxes and whether or not that would be obvious if one released the overview pages from a year's filings (similar to what Rubio and Cruz did).

"Yes, you would be able to tell that on the second page of the return, absolutely," Goldburd said. But to know why a filer wasn't paying taxes, one would have to dig into all of the various schedules attached to the document.

"There are many legitimate ways -- very legitimate ways -- that you can make a lot of money in one specific year and still pay nothing in taxes," Goldburd said.

There are a few ways in which that might happen. Income tax, as the name suggests and as you no doubt know, is tax on net income. That "net" is important: If you earned $100,000 in a year but also lost $100,000 that same year, it's a wash. Your net income is zero, and you don't owe any income tax. (All of this is very much simplified, so please consult your local H&R Block before using this article to rebut an auditor.)

But here's the thing: If you'd lost $200,000, the scenario could be different. "Let's say anybody, John Smith, had a business that went bad," Goldburd said. "If you have a net operating loss, you can roll it over, year-to-year, and it could technically wipe out your taxes this year. If you made an investment last year and you lost a million dollars on that business investment, and this year you only made $500,000 [in income], by rolling over last year's net operating loss, you just wiped out your $500,000 for this year. But not only that, you still have $500,000 to wipe out for next year!" And that holds true for bigger sums over longer periods of time. If you lost $10 billion five years ago but earned $1 billion a year since, you could have paid no taxes for the past five years -- and still have $5 billion in credit (so to speak).

There's another layer when it comes to real estate.

Let's say you buy a building for $10 million. Every year, the value of that property depreciates. Say it depreciates $1 million a year. (The IRS mandates such a depreciation schedule.) After year one, it's worth $9 million. After year two, $8 million, and so on. After nine years, it has depreciated $9 million. If it were sold at that point for more than you paid -- say, $11 million -- you'd pay taxes on the $10 million difference between the $11 million you got and the $1 million value to which it had depreciated.

But there's benefit! That $1 million a year counts as a loss. So if you make $10 million a year in income, but also own 10 properties that are each depreciating at $1 million a year, your income and your losses are equivalent. Boom. No net income, no income tax.

We don't know what Donald Trump's income taxes look like, of course, and we don't know how much his properties are depreciating each year. (We reached out to the Trump campaign to answer these questions, but have not yet heard back.) Even if he released the first two pages of his returns, we wouldn't know the details of why he paid whatever he paid in taxes -- just the top-line numbers of what he did or didn't pay. The details of depreciations and rolled-over losses and all of the things that would release his tax burden would be found in many, many pages of schedules and appendices.

Say, three feet worth.