For years, like the Bermuda Triangle, it attracted curiosity but was never actually seen. People swore they had passed through it on the way to their appointed destinations but could not recall any solid evidence of its existence.

But now Arlington County's Pentagon City promises to pass from the realm of myth to the reality of masonry.

Until recently little more than an out-of-place subway stop in the midst of empty lots, the 116-acre tract will be the site of what is described as the largest mixed-use development in the country. It is located on Arlington's biggest remaining undeveloped site, wedged between Crystal City and the Pentagon just off Shirley Highway.

When completed, it is to have three or four office buildings, highrise apartments, hotels and a shopping mall with three or four large department stores. The developer, Rose Associates of New York City, says that the entire project should be completed in about a decade.

A major boost for the developer's plans came with the recent decision of MCI Communications Inc. to consolidate some of its operations at Pentagon City and eventually to occupy all of the office space at the site, more than 1 million square feet now planned to go on one block.

After months of intense negotiations and a rezoning of the site by the Arlington County Board to accommodate the agreement, MCI signed a contract with Rose Associates last week to buy nine acres of land and two office buildings for $68.5 million. The first 240,000-square-foot, 12-story office building was completed last winter, and construction is to begin on the next 290,000-square-foot building in July. As the company expands, MCI will be able to build another 515,000 square feet on the same block, bounded by Army-Navy Drive, South Fern and South Hayes streets, and 12th Street South.

"We very much wanted MIC," said Daniel Rose, president of Rose Associates. "It was the right kind of organization: a high-technology company with highly skilled, professionals" as employees -- and potential customers for the planned shops and stores there. MCI plans to start moving employes to the new office in November.

It is unclear when the next phase of development, after construction of the office buildings, will begin, but Rose said he hoped to be able to announce plans for the shopping mall and hotels this summer. He said he is negotiating with major department stores and hotel chains. The mall is to have 800,000 square feet of retail space, most of it going to the department stores but with about 300,000 square feet for smaller shops, Rose said. The plan calls for a total of 2,000 hotel rooms.

Asked about a report that Saks Fifth Avenue and Bloomingdale's might put new stores in the Pentagon City mall, Rose answered coyly: "It would be premature to discuss it -- but you can note the grins on our faces."

Lawrence Hill, a spokesman for Saks, said the New York-based, high-fashion retailer "has been looking at the possibility of putting another store in this market at some time" but that he could not confirm whether Pentagon City was being considered. "I have heard downtown; I have heard Tysons Corner; I have heard Fair Oaks," he said.

Saks now has only one store in the Washington area, located in Chevy Chase, and it is one of the most profitable in the country, Hill said.There is no room to expand at the current location, and opening a new store in Northern Virginia would make sense, he said: "It's just a matter of where and when.

Bloomingdale's had no comment on the report.

In the meantime, residents in the area have some concerns about the changes in the Pentagon City site plan, which, while not increasing the amount of office space, shifted all of the office buildings to one block rather than being spread over three different parcels.

"We are very pleased to see progress going forward in the Pentagon City area," said Lincoln C. Cummings, president of the Aurora Highlands Civic Association. "We are concerned, however, that the reallocation of density could well create another Rosslyn" -- a reference to the virtually unrelieved concentration of high-rise office buildings jammed into the Northern Virginia site just across the river from Georgetown.

Specifically, the residents worry that the developer may be backing away from the mixed-use concept and could create an office-building complex that is dark evenings and on weekends and might not be safe for pedestrians, Cummings said.

"We learned a lost from Rosslyn. It's a great place to work but not to live and play," Cummings said. "We have worked on the mixed-use idea for years. It would be a tragdey if business [considerations] allowed the developer to back off from the mixed-use concept now."

But Rose vows that the mixed-use idea is still uppermost in the developer's mind and also uses Rosslyn as the example of what they hope to avoid most.

"There is no sense of cohesiveness at Rosslyn," Rose said. "This will be better planned." Rose contends that Pentagon City is a "state-of-the-art" development, with all components intended to relate to one another both by architecture and access. There are to be tunnels linking the Metro stop with the shopping center and the MCI building.

"The whole idea is to create a sense of presence, a sense of place," Rose said.

The entire Rose development is bounded by Army-Navy Drive, South Joyce Street, 15th Street South, and South Eads Street.

It is to include 5,500 apartment units in high-rises on different parcels at the site. Rose said he hopes these can be built as rental units, but that condominium ownership remains a possibility.

"I would rather build rental housing. We can always build condos, but there is a crying need in the Washington market for rentals," Rose said. But he added that they would "play if by ear" on the final decision.

The Rose development is part of a larger Pentagon City area that was acquired for $1.5 million in 1946 by developer-builders Morris Cafritz and Charles H. Thompkins. One parcel was given to Arlington County for a park. Another chunk of the Cafritz-Thompkins tract includes the Southampton town houses, the Claridge House high-rise apartments and the Carriage Hill nursing home. And in the middle of it all is a Western Electric Corp. plant.

To accommodate the increased traffic from the development, most of the streets in the area are to be widened.

Cummings of the area civic association sees the widening plans as a "step in the right direction," saying that residents have been concerned about the traffic flow when Pentagon City is fully developed.

Dan Rose, meanwhile, called his Pentagon City project "a labor of love." It will be only the first of other developments he wants to do in the Washington area, he said, though he declines to talk about what he might get involved in next.