The limo that carries President Obama down Pennsylvania Avenue to the White House today is the latest in a long line of Cadillacs to join the First Fleet over the years. Although GM and the Secret Service jealously guard the vehicle's specs and secrets, you can bet Obama's ride is the toughest, most sophisticated car anywhere. Think of it as the road-going equivalent to Air Force One.

"Although many of the vehicle's security enhancements cannot be discussed, it is safe to say that this car's security and coded communications systems make it the most technologically advanced protection vehicle in the world," Nicholas Trotta, assistant director for the Secret Service Office of Protective Operations, said in a statement (.pdf).

GM had even less to say, noting "one of the specifications is we don't talk about the specifications." But spokesman David Caldwell says the redesigned car "is a fresh, more modern, more expensive" version of the Cadillac DTS that has carried President Bush since his second inauguration in 2005. Obama's car, he says, is "a little bit more vibrant, if you will."

Even if no one's got anything to say about what's under all that armor, there are some things widely believed to be known about Caddy One's security measures.

The car is one of a small fleet of what is believed to be no more than 25 presidential limos General Motors built for the Obama administration, according to the Detroit News. Although presidential limos have a lifespan of about a decade, the Commander in Chief gets a new one about every four years. Hand-me-downs are used to carry the vice president and visiting heads of state.

GM says the car occupies the "same footprint" as the current presidential ride, but it is a little taller and the windows a little bigger to improve visibility. Limo One is believed to weigh between seven and eight tons, and spy shots suggest it rides on a GM medium-duty truck chassis propelled by a diesel engine. The body is sheathed in military-grade armor as much as 8 inches thick on the doors (each of which weighs as much as the cabin door on a Boeing 747, Motor Authority says). The armor reportedly is a mix of dual-hardness steel, aluminum, titanium and ceramic. The windows are ballistic glass said to be 5 inches thick, and Dan Neil of the Los Angeles Times says there's probably a woven Kevlar mat covering the floorboard to protect the car from blasts. The cabin is believed to feature a sealed air recirculation system to protect its occupants from chemical attacks.

"I think he will be surprised about how when he's in the limo, it's a cocoon," Joe Funk, a retired Secret Service agent who served as President Clinton's driver, told CNN. "The everyday noises will be gone, and he will be totally isolated in this protective envelope.

Still, despite being at least as secure as a hardened missile silo, GM says Caddy One features the same hand-sewn leather interior you'd find in the CTS at your local dealership, and supposedly it's got a 10-disc CD player. And, of course, it's packed with the latest communications technology.

"I think he will be surprised at the communication capabilities, how the phones, the satellites, the Internet - everything is at his fingertips," Funk said. "So at one end, you are totally removed from society. The other side of the coin is that he can have any communications worldwide at a moment's touch."

Obama's limo is the latest in long line of presidential cars Cadillac has built in the 100 years since Congress approved funding for a presidential motor pool. President Wilson rode in a Caddy during a parade marking the end of World War I. President Coolidge sported a lavish 1928 Cadillac Town Car, which was among the first Caddys parked at 1600 Pennsylvania Ave. It featured a 341 cubic inch engine and a robust (for the time) 90 horsepower.

In 1938, the US government received two Cadillac convertibles, each 21.5 feet long and weighing nearly 8,000 pounds. They were dubbed the "Queen Elizabeth" and the "Queen Mary" after the ocean liners. Each featured back-up generators, two-way radios and an arsenal of weapons. They served presidents Roosevelt, Truman and Eisenhower and were replaced by the Queen Elizabeth II and Queen Mary II, which remained in service until 1968.

Cadillac returned to the White House in 1983, when President Reagan rode around in a Fleetwood famous for being the last equipped with the Turbo Hydra-Matic 400 3-speed automatic transmission. President Clinton rode in a Fleetwood Brougham powered by a 454 cubic inch (7.4 liter) V8.

The largest collection of presidential limos is held by the Henry Ford Museum, but President Bush's limo will not appear in it. The Secret Service has since the terror attacks of Sept. 11, 2001, required that presidential limos be destroyed upon retirement to preserve their secrets.