This is a partially non-canon (one can always hope it will become canon) background of the Loop, blending facts from the books with my own guesses and extrapolations.
The Mälarö Facility, or "The Facility for Research in High Energy Physics" (commonly referred to as "the Loop"), was built by the newly formed Riksenergi (National Energy Agency) together with state-owned AB Atomenergi (Nuclear Energy, Inc) on the island of Munsö, some 26 km (16 miles) north-west of downtown Stockholm. Construction began in 1961, combining the best of Swedish engineering and know-how. Tunnels were blasted deep into the bedrock, creating the enormous loops for the world's largest particle accelerators, centered around a huge chamber housing the Gravitron. The facility was powered by the Bona reactor, a subterran nuclear powerplant whose most prominent feature was the three huge cooling towers, the tallest rising 253 meters above the rural surroundings. The light water reactor and major particle accelerator parts were supplied by Asea AB.
The scale of the Loop – or rather Loops – was daunting. The main particle accellerator, Prim-1, had a diameter of 26 kilometer (81,5 km in circumference), unsurpassed to this day. It was three times the length of the most well-known accellerator today, the Large Hadron Collider. Then there was the secondary accellerator, Prim-2, at 20 km in diameter, and the auxilliary accellerator, Aux-1, with a diameter of a "mere" 16 km. Add to this the access tunnels and other underground installations, and we have one of the most impressive engineering feats ever seen. It isn't known how a small nation like Sweden could afford such a massive undertaking, as large parts of the project's budget was classified, but there's speculation that private investors, international corporations, universities, and the US government provided funds in exchange for access to the facility and the results of the research conducted there.