So, the entity comes to light in act one. While Caz is being yelled at by his power drunk boss, Rennick, a worker going by the name Gibbo calls up to say there's an issue with the drill, something highlighted earlier by another worker. Rennick orders the drilling anyway, and thus begins the nightmare, as the drill seems to unearth and awaken a destructive parasite out for revenge. But I don't think that's as deep as it goes, pun intended.

Let's say, the creature is a parasite. A form of near sentient bacteria, though take that description with a grain of salt, I'm no scientist. Parasites simply cannot live on their own. That's an objective fact. They need a host. They exist within another for survival and breeding purposes, and multiply and spread through the body of another.

Parasites, bacteria and even fungi can live underground for years, and have been discovered to do so. Ancient lifeforms have been discovered just under the surface of earth, let alone deep underneath the ground and in the bottom of our oceans, one of the most complex and diverse biomes that our current science has barely scratched the surface of. It's highly likely this creature is a self replicating bacteria or parasite that was unearthed by the drill, and took up new hosts to survive in this change of environment.

But not every host worked. You can see half transformed, mutilated bodies everywhere, and some that haven't even changed at all. These bodies could not support the parasite and shortly died.

However, a few select hosts DID end up surviving; Gibbo, Muir, Rennick, Addair and Trots. It's unclear what sets these people apart from the others, and I don't know enough about this topic to claim an answer. But I certainly do think these folks died soon into the transformation, and are not fully conscious in the body. They frequently repeat terms and phrases, and never say anything you might expect from an entity possessing them, implying it's borrowing words and sentences that have been said by the host before, in other circumstances.

The entity plays with Caz's memories and definitely the others' too, though not all of them good. It wouldn't be a surprise to realise that's where it's getting information about it's host, as it reads the memories inside the brain to learn faces, names, and even the host's personality. Which makes me wonder, does it even realise what it looks like? Does the creature itself actually realise it's a parasite? Or does it completely and fully believe it is the person it's connected itself to? It almost downloads their personality and tries to pretend like everything is completely normal.

Let's talk about arguably the best monster (in my opinion), Muir. Muir moves about the area he frequently worked as what I assume was an engineer. He roams the familiar ground, almost unsure of why he's by himself. He often calls out to his coworkers, wondering out loud why they're treating him like he's different. Sure, this could be the real Muir's consciousness slipping in and out of the seams, but it's highly unlikely he would still be alive. Much like the zombie fungus, as it's often called, the host is not alive when the fungus is controlling it, and is merely a puppet. If the spiders it was corrupting could talk, I daresay, they'd be acting like them. Taking their place in the world, even if they don't realise it.

But every animal needs to eat. And eventually, that body is going to run out of tasty, tasty neurons. Like I said earlier, a parasite needs to spread. It'll breed, then spread to another to keep it's species alive. By infiltrating a 'pack' of animals, it will take anything to spread to the others. Which is exactly what the parasite does whenever it sees another human. Either that, or it will consume them, theoretically to feed the host so it stays alive, while keeping those tasty, tasty neurons for itself. You can almost see this process with Innes, as the elevator ascends without him, and you just faintly see Muir doing something in the distance. Likely consuming him for nutrients, as he was not connected to the parasite yet.

Next, there's Addair.

Addair, much like Muir, patrols familiar ground. Even though Addair himself wasn't even in that area when the drill struck. Now Muir was actively in that familiar space in the beginning, and it's safe to assume that's his place of transformation. But Addair was eating in the cafeteria when the incident happened, not deep down in the engine. Did he go down when the impact happened, while Caz was unconscious? Maybe. But the lights were fine then, and the engine wasn't the problem, so he didn't need a reason to. Plus, he doesn't seem like the type to be work dedicated, more inconvenience dedicated. Considering what I said about the parasite (badly) taking their place in society, did it go to his place of work after detecting that as his 'natural environment', per se?

Plus, unlike Muir, who greets the situation with quotes of confusion, fear and anxiety, Addair is instantly aggressive. Even an asshole like Addair is likely to panic if conscious in this situation, so the nervousness was Gibbo and Muir exclusive. But Addair and Rennick become immediately angry upon seeing Caz, as they actively disliked him in life, and so the parasite processes him as a foe to it's host. I thought that was neat.

Now another take I have admittedly heard from several other people, but I thought was worth mentioning. The monsters are incredibly similar to sea creatures. Which means this underwater bacteria was possibly leaking out already, and transforming our animals, not enough to completely corrupt them, but enough to twist their bodies. Think of the appearances of deep, deep sea creatures, such as the anglerfish. Isn't it possible this parasite was responsible for their uncanny appearance, in this universe? Muir especially looks like a spider crab, or perhaps even a bigfin squid.

Which again, is a deep sea creature. Rennick also reminds me of a blobfish once removed from the pressure of the deep sea. Addair seems very jellyfish-like, but may be something else very... tick-like. And even Trots gives me major merfolk vibes, with how untouched his torso is in comparison to his lower half.

This parasite could have been feeding off the neurons and breeding through our very ecosystem as the ground slowly gave away above it. The drill unearthing the source likely gave it a burst of control as so much energy was released at once, hence why it was so fast to literally spiral out of control.

But Scotland, by all means, is not the only place in the world connected to the ocean. Sure, they destroyed this batch, but other forms of this parasite live on elsewhere on earth. And the explosion may not have even destroyed it. It definitely would've destroyed the host bodies, yes, but certain bacterias can survive impressive damage, even heat hot enough to burn off human flesh. We'd best hope this is not one of those bacterias.

I didn't really get as far with this observation as I did with other horror studies, but I had fun nonetheless! Like I said, I'm really better with zoology (hence the sudden enthusiasm when I started on sea creatures), but I loved Still Wakes the Deep SO much that I just wanted to write down my thoughts. If you have any other theories, feel free to add them!

Also if I used your pictures/gifs and you would like me to add credit, I am so so sorry, I will absolutely add that as soon as you say so, I just got most of these off Google and couldn't find most the original sources. So yeah if you'd like me to add your name and mention, or you want me to remove it in general, feel free to just say and I'll add it, I don't bite I promise. Well... I won't bite YOU.