We know a thing or two about off-road vehicles. Or at least we thought we did, until Mercedes-Benz invited us to Milbrook Proving ground in Milton Keynes to have a go in some of its most terrifying capable all-terrain vehicles.

Upon arrival, we were greeted by a Mercedes-Benz G-Class, a Sprinter 4x4 and two beasts we never dreamed anyone would be foolish enough to let us have a go in -- the gigantic Unimog and the horrifying wheeled spectre of doom that is the Zetros. Needless to say, we took great pleasure in hooning around in all four, and we took some pictures of our adventures to share the experience. You're welcome.

G-Class

G-Class

We started our off-roading adventure in the smallest of the four vehicles, the G-Class. This bad boy, a favourite of rappers and Hollywood stars, proved itself to be as capable as it is box-shaped, and as you can see, it's about as boxy as they come.

The key to its capabilities are its three locking differentials. If it sounds hugely complicated, that's because it is. But, in a nutshell, it allows the G-Class to turn all four of its wheels at the same speed -- even if one wheel is on a slipperier patch of ground -- thus guaranteeing the best traction.

We can't claim to understand the ins and outs of the thing, but it boils down to this: flick the three dashboard switches for the front, centre and rear differentials and the G-Class will make mincemeat of any terrain. The way it tackled muddy, near vertical slopes, both up and down with so much control, it seemed almost to defy physics.

If, for some reason, you do get stuck halfway up a slope, there's no reason to panic, as the G-Class' cabin is a thoroughly lovely place to hang out. Our test vehicle was fitted with red leather seats, a hard drive-based sat-nav, Harman Kardon speakers and a rear seat entertainment with twin screens and individual DVD players.

G-Class specs

Engine: 2.9-litre V6 Diesel
Transmission: 4WD with front, rear and centre diff locks
Power: 224bhp
Price: £80,000
Mud rating: 4/5

Sprinter 4X4

Sprinter 4X4

We hopped triumphantly out of the G-Class, into a Sprinter 4X4 and steeled ourselves for the worst. To say we didn't have much faith in this thing's off-roading prowess would be an understatement. It is, after all, a Sprinter, and that's a van, and vans are generally only good for two things: transporting cardboard boxes and terrifying cyclists.

Setting off, our fears were compounded by the Sprinter 4x4's smooth ride. Barring the lack of sound deadening in the cabin, it was arguably a nicer drive than the G-Class. Once we got over the fact it's so enormous (it rides 11cm higher than the standard Sprinter) we could be forgiven for thinking we were in a more car-ish Merc.

Once we got near the muddy stuff, we were instructed to come to a standstill and to hit a small button on the dashboard to engage the big Sprinter's 4ETS Electronic Traction System, after which it went about its off-roading business like a vehicle possessed.

It went absolutely anywhere the G-Class went. It looked and felt a little ungainly at times, cocking its wheels in the air on uneven terrain, but it was thoroughly capable, even managing to conquer hills made of deep, soft sand -- the off-roader's arch nemesis.

The Spritner 4x4's all-wheel-drive gubbins is totally different to the G-Class. Whereas that vehicle uses mechanical differential locks to keep all its wheels spinning at precisely the same rate, the Sprinter 4x4's 4ETS system is based essentially on the electronic stability control (ESC) setups found in most modern cars.

Its power transfer unit allows the engine to drive all four wheels, but when the minute on-board sensors detect one wheel is slipping and spinning faster than the others, it applies the brake to that individual wheel, slowing it down to help regain traction while continuing to channel torque to the wheels that still have grip.

Sprinter 4x4 specs

Engine: 3.0-litre V6 Diesel
Transmission: 4WD with electronic traction system
Power: 190bhp
Price: £80,000
Mud rating: 3/5

Unimog U500

Unimog U500

Having hopped out of the Sprinter 4X4, it was time to tackle previously unseen parts of the Milbrook facility. These sections, designed to test dedicated all-terrain vehicles, would be a bridge too far for the G-Class and Sprinter 4X4, but they proved to be childs play for the Unimog.

It's a bit of a beast -- and that's putting it mildly. This thing, which looks like a giant Tonka toy crossed with a monster truck, was designed originally to operate harvesting machines in fields, or saws in forests, but it was quickly adopted for just about any task that requires traversing extreme terrain. Fancy carrying military troops into harsh jungles, embarking on rescue missions in inhospitable deserts, or trying to negotiate the treacherous roundabouts of Milton Keynes? This thing is all you need.

We tried a couple of Unimog variants, starting with a model that had been converted into an ambulance. It felt, to all intents and purposes, like we were driving a four-bedroom house -- albeit one that can drive over other four-bedroom houses.

It's probably a big enough beast to shunt all obstacles out of its path, but it actually uses some sophisticated technology. It has a locking differential that guarantees non-slip power transmission to all four wheels, telescopic shock absorbers that force the wheels downwards into the surface of the road to increase grip, a strong but flexible chassis that twists with the terrain rather than fighting against it, and tyres that can inflate and deflate themselves on the move to improve traction by as much as 30 per cent.

What surprised us most about the Unimog was the fact it's not quite as agricultural a drive as we'd imagined. The eight-speed sequential gearbox was as easy operate as any sports car -- you simply push the stick forward to shift up and backwards to shift down.

Unimog U500 specs

Engine: OM 906 LA Euro 4
Transmission: 4WD with electronic traction system
Power: 286bhp
Price: £90,000
Mud rating: 4/5

Zetros

Zetros

We walked away from the Unimog thinking we'd seen the ultimate off-roader, but we were wrong. That honour, where the Mercedes fleet is concerned, belongs to the Zetros. This thing is purpose-built and combines the off-roading capability of a Unimog with the load-lugging capabilities of a lorry.

Like the Zetros, it'll go absolutely anywhere, which is remarkable as it weighs a staggering 27 tonnes. Obviously, this can present problems, particularly when driving down steep, slippery hills, but the Zetros (like the Unimog) has some clever on-board tech that prevents it from turning into a death toboggan.

It uses an exhaust brake to slow the engine. Hit a button on the dash and this system closes off the exhaust path from the engine and cuts the fuel supply, which causes exhaust gases to be compressed. Because the exhaust is being compressed and no fuel is being applied, the engine effectively works backwards, slowing the vehicle instead of accelerating it.

With the wheels turning super slowly, the Zetros is able to roll slowly down the steepest of hills, its wheels turning constantly to keep it firmly under the driver's control.

The Zetros isn't all about going slowly, though. It packs a massive 7.2-litre engine that produces 326bhp, which means it can travel at a fair rate of knots over rough terrain. Have a look at this video of a Zetros mercilessly hunting down a G-Class and other puny 4x4s to see what we mean.

Zetros specs

Engine: 7.2-litre
Transmission: 4WD or 6WD
Power: 326bhp
Price: £POA
Mud rating: 6/5

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