Iron Mountain (NYSE:IRM) is something of an anomaly among real estate investment trusts (REITs). There is, quite literally, no other REIT exactly like it, for reasons I’ll explain shortly. Now add in 10 years’ worth of annual dividend increases and a roughly 8% dividend yield, and income investors should be intrigued. Here’s a primer on what you need to know before you decide to pull the trigger on this high-yield REIT.What’s particularly nice about this business is that once a company puts boxes of paperwork into Iron Mountain’s care, it very rarely wants them back. And companies certainly don’t want to take their boxes and move them to a new provider. To put some numbers on that, Iron Mountain has a 98% customer retention rate, and roughly 50% of the boxes for which it is responsible remain in its care for at least 15 years. This is a sticky business. This core physical-world operation is helping to fund an expansion into the digital realm — so Iron Mountain isn’t a one-trick pony, and it isn’t sitting on its laurels. It is using its existing connections with business partners to sell the data center services it is building out. Look at it as providing storage in a new, high-tech way. It currently has 14 data centers located in key markets around the world. Over time, investors should expect this new business to become Iron Mountain’s main business. That said, with the long-term nature of its physical storage operations, a full digital makeover could take a very long time. This isn’t a bad thing, however, because it means that Iron Mountain can take its time, and there’s plenty of leeway for minor missteps along the way.