And should there be a leak, there's both a controlled venting and rapid venting process to let the hydrogen roam free again. There's a big vent on the roof as well as one on the underside, so no matter which way the car lands in an accident, the hydrogen goes out the higher vent. (I forgot to ask what happens if the H7 lands exactly on its side.) While gasoline puddles on the ground, hydrogen, being lighter than air, goes straight up. Inside the trunk, engine compartment, and passenger compartment are five sensors that detect even minute hydrogen leaks. If there's a leak, the translucent door lock buttons blink red - it's hard to miss. (I'm told.)

Another practical impractical BMW
BMW has a tradition of building the most practical of impractical cars. Its 6 Series luxury sports coupe runs $75,000-$105,000 and is classified as a 2+2. Yet the trunk holds a pair of golf clubs plus 2, 3 or 4 pieces of soft-sided luggage - no need to FedEx your luggage ahead -- and the back seat is actually pretty comfortable for those less than six-feet tall. Impractical, yet practical, unlike say a Pontiac Solstice with trunk room for two tiny gym bags. In the case of the Hydrogen 7, the car defaults to running hydrogen fuel, yet you can switch to gasoline at the touch of a steering wheel button, and extend your 150-mile hydrogen range by another 300-plus miles. Most hydrogen prototypes, such as the GM Equinox, are hydrogen-only and once the hydrogen tank goes empty, you call for a flatbed. As I said, the BMW is practical.

How it works: Technology behind the Hydrogen 7
The car starts life as a 12-cylinder BMW 760Li ($124,925-$133,655) with its stretched rear seating area. BMW plumbs the car for liquid hydrogen, which it burns as an internal combustion engine fuel, rather than using the hydrogen to create electricity that spins a motor. Either way, the only emission is water vapor. And what comes out the exhaust pipe of the Hydrogen 7 is actually cleaner than the surrounding air, since airborne pollutants coming in with the air supply are either incinerated or trapped by filters.

To make it go, you press the start button (then release) and wait for two or three seconds until the engine catches. It always starts on the cleaner-burning hydrogen and runs for 2-3 minutes on hydrogen, until the engine warms up, at which point you can switch to gasoline by tapping the H2 button on the steering wheel. Listen carefully and you'll hear a valve behind the rear seat shut off the hydrogen flow. The two modes sound nearly the same. The hydrogen process causes a bit more clatter because of the rapid opening and closing of valves that let the now-gaseous hydrogen flow into the engine at about four atmospheres (roughly 60 psi) whereas gasoline is directly injected. Open the well-soundproofed hood and the clatter becomes a loud clatter. During full acceleration, you can also hear more engine noise. A bit more.

BMW stores the fuel as liquid, not gaseous, hydrogen, and as you probably recall from high school physics, a liquefied fuel packs more power than a gaseous fuel, but to keep it liquid, you've got to keep it colder. Despite that, half the spacious trunk plus a couple inches of back seat room is given over to the fuel container, which is done up in the shape of the national vessel of Germany: a beer keg. This is a keg inside a keg, with a vacuum between the two shells, so the outside surface is at air temperature while the 8 kilograms of hydrogen inside are at minus-320 degrees. The insulation from the outside is very good but not perfect, and small amounts of hydrogen become gaseous and a combined in a mixing area with oxygen, plentifully available in the outside air. It's released with a slight hissing sound. Rattlesnakes in heat will love the Hydrogen 7. Over the course of a couple weeks, the liquid hydrogen tank would vent itself. In other words, if you can't drive at least 100 miles every two weeks, this car isn't for you.

Non-technical aspects
The Hydrogen 7 is about as luxurious as a car can be, based on BMW's top-of-the-line 7 Series (that is soon to be replaced by the fifth-generation 7 Series. It's got built-in navigation, built-in iDrive both front and rear (with slippery control knobs that BMW changed in other models), rear seat DVD, and night vision. The front seat backs adjust every way imaginable; you can adjust the rake of the top half of the seatback independently from the bottom half. The 5 back seat inches of legroom gained in the long (L) version of the 7 Series are lost to the hydrogen tank, hoses, and safety gear, but it's still spacious.

Should you buy? Um, can you buy?
The buying decision has been made for you. You can't buy one. BMW's fleet of 100 or so Hydrogen 7s are for short- and longer-term use by influencers. In addition to show-biz types who bring BMW plenty paparazzi coverage, they've also been in the hands of state and national politicians, environmentalists (Inconvenient Truth director-producer Davis Guggenheim), journalists and analysts, as well as some hand-selected mainstream drivers, typically those who've owned BMWs in the past.

Anyway, right now hydrogen is ridiculously expensive to buy and there are only a handful of limited-access stations, mostly clustered in the Los Angeles, New York, and Washington areas. Add in the limited range of hydrogen vehicles (non-BMW hydrogen vehicles don't have fallback propulsion) and this is a fascinating glimpse of our future 5-20 years from now. What's most impressive about the Hydrogen 7 is that it's utterly normal.