Although we don't believe in timing the market or panicking over market movements, we do like to keep an eye on big changes -- just in case they're material to our investing thesis.

What: Shares of property trust Great Northern Iron Ore Properties (NYSE: GNI) shed as much as 25% in intraday trading on heavier-than-average volume.

So what: The newswires were quiet regarding Great Northern today, so the reason for the hefty decline remains relatively speculative. Just last week, I took a close look at the stock and found that, due to the fact that the trust expires in just a few years, the stock is likely radically overvalued. While I have yet to hear from an investor that's been able to short the stock, today's action could be the result of current shareholders realizing that the current high price makes now an advantageous time to sell.

Now what: Today's drop seems to be a move in the right direction in terms of getting the stock back to a sensible valuation. However, all signs seem to point to the stock being worth even less than the $104 that it's trading at as of this writing. Investors that are sniffing around Great Northern would probably be well advised to continue to stay on the sidelines for now and wait for the price to reach a level that is justified by the likely payouts during the company's remaining life. Either that, or they could simply move on to greener pastures .

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Fool contributor Matt Koppenheffer does not own shares of any of the companies mentioned. You can check out what Matt is keeping an eye on by visiting his CAPS portfolio, or you can follow Matt on Twitter @KoppTheFool or on his RSS feed. The Fool's disclosure policy prefers dividends over a sharp stick in the eye.