Description: take a M26 Pershing tank, replace the 90mm main gun with a stubbier, classified cylinder which causes nosebleeds in any human that looks too closely at the classified symbols etched on its surface, swap out the engine for a literally indestructible cube of some classified alloy that smells faintly of copper and iron, and cover the (significantly reduced) armor with more alarming, classified symbols. The machine guns remain unchanged. The sealed crew compartment is larger than a regular Pershing tank, and can carry 6 humans and their gear sustainably. Hellcrawlers have a classified life support system that can keep up to 9 humans alive indefinitely.
Weight: 45 tons
Operational Range: For as long as the crew holds out
Max Speed: 50 mph on roads, 10 mph off road
Crew capacity: 6
Hellcrawlers have actually gone to Hell and back. Or they have gone to a Hell. Whether it was the Hell is a question for theologians, or possibly the United States State Department. It turns out that access to other dimensions creates more questions than answers.
There are other questions about Hellcrawlers, and the answers are all classified. What is known about them is that they’re based on something that American researchers stumbled upon in 1943, but couldn’t get to reliably work until after World War II. This something seems to allow access to other dimensions, among other things; but most of those places are decidedly hostile to human life. It’s reasonable to assume that various elements of the nascent military-industrial complex – including the elements that show up on no invoice, org chart, or possibly even periodic table – thus got together to create the first Hellcrawlers, in order to let the USA explore and scout these dimensions in relative safety. Or at least increased safety.
There aren’t many Hellcrawlers, because they were extremely expensive to make; but each one is incredibly tough. As long as the cube that serves as an engine is hooked up to the chassis, the vehicle is highly resistant to damage, and will eventually ‘heal’ itself of its injuries. Life support is also indefinite, but has an upper limit. A Hellcrawler probably wouldn’t be able to survive on a plane of pure, crawling chaos – but ones have gone to frozen plateaus of sorcerous terror, or blasphemous cities overlooking stagnant lakes reflecting twin moons, and come back. Usually they even bring back the crews safe and reasonably sane, too. As even a cursory look at the literature of interdimensional exploration can tell you, this is no small feat.
Hellcrawlers are not something adventuring types might acquire; they are items that are assigned. If there’s a need to traverse a dangerous otherworldly space, and the US government is called upon to assist, something like this is what they’d cough up. Actually securing one for permanent use… has been tried. Unfortunately for the people who tried it, it turns out that how to keep a Hellcrawler functional is classified, too. And the people who are cleared for that information are worryingly tight-lipped about the details. As in, maybe you don’t want to know; and they definitely don’t.