Why the Graboids from "Tremors" Totally Would Not Have Worked, and the One Thing They Got Right, Part I

The move to California has made its impact on my life, and consequently this blog. I give my employer credit for keeping me busy, which severely limits blogging time. On the other hand, it has given me ample exposure to Quaternary alluvium (Qal), and much time to ruminate on this type of sediment. One of my most consistent ruminations revolves around the monsters from one of my favorite movies - the 1990 cult classic “Tremors.” Unfortunately, my armchair evaluations have led me to conclude the monsters, dubbed “graboids” in the movie, would never work realistically. For this blog post, I will begin explaining how I arrived at this conclusion…

The monster of concern, in true 90's-era video quality.

NOTE: In my “research,” I tried to exclusively limit myself to data presented in the original movie, and not any of the lame sequels or fan fiction. Apologies if any of the following criticisms are resolved in subsequent movies.

cinemajunkyard.wordpress.com/2011/10/07/tremors-1990/

For those who have not seen the movie, the plot revolves around the denizens of a small, isolated, rural Nevada town trying to escape the wrath of four giant carnivorous subterranean worms. That’s not the best sell on my part, but it works on the screen – I would recommend watching it for those that have not seen it.  

First, some background on the monsters: no specific dimensions are ever given for the graboids, but the picture to the left indicates they are around 2 m wide, and the occasional full-body shot in the movie suggest they push 10 m in length. For comparison, that is slightly larger than an orca (this will be important later). Nearly all the necessary information about the graboids’ behavior is nicely summarized in the following clip from early in the movie, when “Old Fred” meets his demise:

As you can see, graboids live and travel underground (technical term: fossorial burrowers), and not very deeply - their arrival in the above clip disturbs a scarecrow that is anchored into the ground by no more than a couple feet. Additionally, throughout the movie, graboids are frequently shown to be traveling less than 1 m below the surface, especially when stalking or capturing prey. A key plot point in the movie is the graboids can only travel through the Quaternary alluvium – when one runs into a ~50 cm cement wall, it ends up killing itself through blunt force trauma. They also capture their prey by pulling it underground as Kevin Bacon wryly observes (warning: simulated horse mauling). In the movie they are shown (or implied to) pull sheep, humans, horses, and even an entire station wagon underground.

Second, some background on the author: I am currently employed primarily as a “paleontological field monitor,” meaning I watch construction, much of it in Qal, for the disturbance of fossils. This presents the opportunity for long periods of deep thinking, but not much else to envy. Conveniently, much of the groundwork I have observed is comparable to what would be required of a whale-sized fossorial organism, and I have arrived at two major flaws with the graboids.

Problem #1: No, you’re a dense medium!

First off, let’s return to that “fossorial” part. In contrast to the more familiar terrestrial and arboreal (flying) animals that live and travel in air, and aquatic organisms that live and travel in water, fossorial organisms spend at least part of their lives in the soil. For the purposes of this discussion, we will sidestep the strict definition of “soil,” and just keep in mind that the majority of soil is loose rock grains. Forgivably, most of us are only familiar with soil when it is in a unique, heavily disturbed state – after is has been dug up and placed in a pile, or well-tilled and aerated in a garden. Under these conditions, the grains have been separated and the soil is spatially unconfined. But in its natural state, soil is under stress from the weight of the soil above it, and confined by the soil surrounding it in all directions. This causes grains to tightly compact, resulting in a heavy, dense medium.

For comparison: the atmosphere through which we regularly move is .001 g/cm³. According to casual conversations with soil scientists, undisturbed Qal like that in the movie is naturally 75 – 80% grains by volume (rock = 2.7 g/cm³, for our sake). "Back of the envelope" calculations suggest the soil itself would have a density of at least 2.3 g/cm³, i.e. over 2000 times denser than air*.

* For a good “hands-on” example, pick up a bag of vacuum-packed coffee grounds at the grocery. In that state, the contents are about 80% grounds by volume, and the vacuum packaging simulates the confinement from surrounding sediment. Imagine digging through something like that, but harder, and you have a good approximation of a burrowing lifestyle.

ossamenta.dreamwidth.org/24900.html

Needless to say, unless an organism is microscopic, traveling through such a dense medium places extraordinary demands on its body. For a classic example of this, look no further than the image at the left – the bone on the left is a rat humerus, and the bizarre-looking bone on the right is actually the humerus of a similarly-sized common North American mole. Evolution has forced it to extensively remodel itself to accommodate enormous arm muscles for a lifetime of efficient digging. The adaptations of other digging organisms – from the massive teeth and neck muscles of mole-rats to the lethal claw of badgers – are all testaments to the demands of living underground. Consequently, fossorial organisms are relatively quite rare – if I were to spontaneously ask you to name all the burrowing vertebrates you could think of, you would probably list a sizeable percentage without trying too hard.

Part of the reason for this rarity is that the extraordinary demands require extraordinary fuel. A single mole or gopher can wreak a seemingly disproportionate amount of havoc on your victory garden because is needs a lot of nutrition to move through something 2000 times more challenging than a hurricane. And even the best burrowers are comparatively painfully slow at subterranean locomotion – I was unable to find estimates for how quickly various burrowing organisms can move while underground, probably because it’s too slow to really warrant a velocity. In contrast, graboids are shown to burrow at speeds faster than a sprinting young adult male human, and travel dozens of kilometers (at least) over the course of three days. And they are whale-sized.

Again, for comparison: the image to the left (traffic cone for scale) shows auger bits used on a highway expansion project I monitored. The auger excavated a 1.5 m-diameter tunnel, roughly equivalent to the width of the graboids. But it took five 8-hour days, running on two 11,000 watt generators, to excavate a 15 m-long tunnel. Now, 22,000 watts converts to 29.5 horsepower (hp). In an interesting paper in 1993, Stevenson and Wassersug calculated the upper limit of an actual workhorse’s power output to be 14.9 hp, and that it could only last for a few seconds. If an organism could double this theoretical maximum and maintain it for hours, it could potentially burrow at 0.3 km/hr – definitely not enough to catch Kevin Bacon in a dead sprint. To account for the activity exhibited in the movie, a single graboid would have to possess magnitudes of order more power than any known organism on the planet, and have a food source rich enough to fuel it. And there were four of them. The handful of people and livevstock ingested would not have been anywhere near enough fuel to go around.

OK, I’m willing to concede the graboids’ burrowing abilities are merely highly implausible, not impossible. One of reasons I like the movie is because it doesn’t make the mistake common to many science fiction movies of trying to explain too much. The origin of the graboids is never determined, so for all we know, they could be aliens with an unknown, exceptionally efficient fuel source – ultra-brown fat, perhaps. But even if they came from another planet, the laws of physics are universal, which brings us to problem number two…

Problem #2: Aristotle, still relevant after all these years.

One other thing you might notice about the fossorial organisms you named earlier is how small many of them are – squirrel-sized burrowers are the norm. There’s good reason for this for this – for any subterranean space excavated, the disturbed soil has to be moved somewhere, and the less dirt to haul, the better. For permanent or semi-permanent burrows and dens, this is a one-time hassle. But, as discussed above, the graboids are shown to be constantly on the move – while hiding from one, a character comments “Doesn’t he have a home to go to?” But nowhere in the movie are visible castings or spoils from the enormous burrows the graboids must have excavated – note how in the above clip, Fred's remains are surrounded by a small halo only a few centimeters high. Even after pulling the aforementioned station wagon underground, there is no station wagon-sized volume of spoils to be seen – two characters only know a vehicle is underground because of the still-active radio. 

cnn.com/2013/03/01/us/florida-sinkhole-explainer

This is not just an aesthetic quibble – it is related to a notable problem with the graboids’ unique mode of predation. In order the pull, say, Fred underground, a graboid needs to excavate a Fred-sized hole directly underneath him, regardless of how the excavated soil is dispatched. Problem is, this is one of those instances where horror vacui is very much true – an excavated subterranean space is living on borrowed time. The weight of the soil and sediment overlying such a space will cause it to ultimately collapse - we see instances of this every day with sinkholes. As a prime example, the picture to the left shows not only the start of a commuter’s very bad day, but what commonly happens when a graboid-sized tunnel is situated too near the ground surface. The graboids in the movie are consistently shown to be moving even nearer to the surface, yet only rarely is there any detectable disturbance above ground, and never anything noticeable to the doomed prey. 

Personally, I believe this plot hole was intentionally “edited out” – there is at least one scene where the actual ground (not a scaled-down set) is disturbed in a manner consistent with a near-surface graboid. The subsequent disturbed earth is carefully edited out of the movie, but the film crew would have unavoidably seen the physical aftermath in real life. Furthermore, in one ironic, humorous scene, a character experiences a brief moment of terror when his foot sinks into a gopher burrow. Yet in other scenes, burrows hundreds of times larger are actively excavated underneath a person, yet go unnoticed and remain sturdy until the person is pulled under. I’ll sympathize with the producers not wanting to foot the bill for renting a backhoe to simulate collapsed tunnels for every attack scene, but I do fault them for trying to “sweep under the rug” a fundamental flaw with their own monster’s lifestyle. 

But wait, we’re not done yet! And it isn’t all bad! But I’ve written enough for one sitting. Look for "Part II," coming soon!

REFS

Stevenson, R. D., and Wassersug, R. J., 1993, Horsepower from a horse: Nature, v. 364, no. 6434, p. 195, doi:10.1038/364195a0