The ground is shaking underneath our feet. This means one of two things: either Bedfordshire is experiencing a rather out of sorts earthquake or an impressively large vehicle is about to enter stage left. Environmental disaster averted for a moment, the latter comes bounding into view - welcome the mighty Mercedes Zetros.

Jaws drop so far they brush the blades of grass that quakes underneath its weight, and there are those around us that look like children the night before Christmas. The excitement is briefly curtailed by the bun fight that ensues; it seems that everyone wants to drive the Zetros, more so than any other vehicle assembled here today by the Mercedes-Benz commercial vehicle department. Offering no exception to the rule, we're quick to put our names forward. And then it happens: my turn. Stood only feet away from the driver's door, even an earlier session in the ubiquitous Unimog hasn't been enough to prepare me. This thing is huge. And despite our puny driving licence, they're about to allow us behind the wheel.

Of course getting there is the first challenge, the ladder-like footsteps taking some of the strain, if not completely easing our path. But unlike the other-worldly cab of the 'Mog, the Zetros looks rather, well plain. It's certainly all very familiar; there are no strange levers, buttons or screens to keep an eye on; just a simple instrument cluster, a sensibly sized steering wheel and a regular gear lever.

Ah yes, that gear lever. The new Porsche 911 may have seven speeds for its manual transmission, but the Zetros, well it goes one better and offers a complement of eight. Actually, with its crawler gear it has nine, but that would just be splitting hairs.

What this means is that there's a whole other shift plane in the conventional H layout to worry about, which is why we attempt to start in third rather than first gear. Two stalls later, that's one to the Mercedes, and nil to me. Mistake rectified by pushing the lever even further away, we set off on our journey into the deepest, darkest and dirtiest that Millbrook's off-road section can offer us.

The first job is not to take anything, including photographers, other vehicles or even trees out - make no mistake, not only is the Zetros long, wide and tall, but it's also so solid that there's a good chance you could run over the hospitality unit and the first you'd notice of your mistake is the lack of luncheon.

Capable, composed and comfortable, you'd never expect a ten-tonne truck to feel quite so fit for purpose. But size familiarised, the Zetros' long wheelbase and massive 20-inch wheels make short work of any bumps in the foreground, proving more comfortable over obstacles than anything else we get behind the wheel of today.

And despite the unusual layout, the manual gearshift makes it far easier and quicker to select the correct ratio for the challenges ahead. Sure there's some recalibration of our brains needed - too often we're in a gear lower than we need to be - but the extra cogs make better use of the narrow power band. Torque is monumental at 960lb.ft, but the 7.2-litre engine does most of its work before other units have barely started breathing properly.

Which means that even the steepest of hills pose no problem for us, the Zetros simply dealing with whatever is put in front of it with the minimum of fuss. If truth be told there are times it feels like we're more likely to plough through each obstacle rather than traverse it, but despite the weight there seems to be little issue with sinking.

And though the 2,530mm width means we need to thread carefully through the pre-made lanes, it also gives great stability. While some of the other vehicles have felt like toppling over is a real possibility, the Zetros feels ever stable - in fact it feels more likely to level the ground out beneath us than sideswipe it.

Then the ground stops shaking. We're back at base, the journey into, round and through Millbrook's off-road section finished. Engine off, seat belt released its time we relinquish the driver's seat for someone else. Door open it's a long way down - not just from the cab but - from the high we're riding thanks to the memories of piloting such a mighty machine.

Key Facts

Model tested: Mercedes-Benz Zetros 1833A 4x4
Pricing: £80,000
Engine: 7.2-litre turbocharged six-cylinder diesel
Transmission: eight-speed manual (with an extra crawler gear), four-wheel drive
Power: 322bhp
Torque: 960lb.ft at 1,200rpm
Fuel tank capacity: 300 litres