You'd have to be crazy to tour wintry Toronto without a coat on. Crazy like a mole.

For two days, I was that creature. Forgoing fresh air, traffic and outerwear, I moved into Toronto's subterranean city. The 16.7-mile stretch of walkways, called the PATH, lies below the downtown financial district and is insulated from any mercury drops. Sticking to the netherworld and its adjoining street-level atriums and skywalks, I was able to shop, eat and even sleep without losing feeling in my face.

Indeed, while others suffered in 14-degree weather, I was enjoying lovely conditions: clear and cloudless in the 70s, with not a hint of precipitation.

"It's really a world underground," says Sousie Tsotskos, a spokeswoman for Tourism Toronto. "Some people go to work, go out to eat, go shopping, go to the cinema, see a hockey game — and never have to go outside."

Toronto residents have good reason to stay indoors. In February, the city's average temperature is 23; in March, it jumps all the way to 32. Days before I arrived, the city was under a "cold alert," which warns folks to limit their outdoor exposure for fear of frostbite.

But don't let the inclement weather scare you off Toronto. There's an alternative — if you can exist without natural light.

Toronto's first pedestrian tunnel appeared in 1900, when the major department store T. Eaton Co. connected three of its properties. In 1917, five pathways were added. Ten years later, the Royal York Hotel (now the Fairmont Royal York) and Union Station were linked, allowing refined patrons to travel from the train depot to the haute-class hotel without mussing their coiffures. The PATH really burgeoned in the 1970s, when a connector was constructed between the Richmond-Adelaide and Sheraton centers, two hives of activity.

Today, the PATH links more than 50 buildings and office towers, many bank- or finance-related, and features more than 1,200 stores and businesses, including boutiques, spas, restaurants and hotels. It also offers access to a number of subway stations and some of the city's bigger attractions, such as the Hockey Hall of Fame and the Roy Thomson Hall, a performing arts venue.

The Ontario capital isn't the only city that keeps you covered. Other urban areas around the world, especially those that suffer from debilitating heat and/or cold, have similar setups (see box). The sealed environments are heavily used by residents and visitors. PATH manager Michael Saunders says more than 100,000 commuters use the system every weekday; on weekends, about 10,000 tourists and weather-sensitive locals wander through the labyrinth.

At first blush, the PATH seems user-friendly. Though the system is not an exact twin of the above-ground grid, it does identify the streets and buildings above; walkways vary from mall-like corridors to ornate bank lobbies to skyways with snatches of sunshine. Color-coded signs lead visitors through the maze. Theoretically, you just have to know your yellow from your blue.

In reality, you need to have an internal GPS. The system seems to have been devised by a drunken urban planner. It wanders and zigzags, then dead-ends before stumbling off in some other direction.

"Those colors — they're just colorful," said the Hilton reservationist who checked me in after I spent an hour looking for the hotel, which can be accessed underground through Adelaide Place. "Sometimes I have to go outside, look to see where I am, then run back inside."

The system's northernmost reach is the Toronto Coach Terminal, the city's bus depot; its southernmost is the Air Canada Centre, home to the beloved Maple Leafs hockey team. Other points of interest sit along the eastern and western fringes, such as the Canadian Broadcasting Centre and the Metro Toronto Convention Centre.

The good news: The PATH is set to expand. The bad news: The PATH is set to expand.

Saunders says plans are in the works to bridge the new Four Seasons Centre for the Performing Arts as well as the Ritz-Carlton, still under construction. "We're going to add another mile," he said. "But we're not trying to have the longest one in the world."