It rolled on top of me, pushing me under the water. I was trapped under a mountain of blubber and muscle

The stretch of coast in Cornwall where I live is studded with beautiful beaches, so I try to take my dog Sully, a staffordshire bull terrier, for a good, long walk most days. Sully’s a rescue dog, and a bit bonkers. He would never hurt anyone, but he can be feisty, so I like to take him somewhere deserted, where he can have a proper leg-stretch in peace.

This January, on a clear, bright Saturday morning, I chose to head for a beach you have to clamber over rocks to get to. It was low tide and the sand was dimpled by large pools and gullies left by the retreating water. I chucked Sully’s ball towards the distant sea and watched him race after it. But instead of seeing his compact frame come pounding back moments later, there was nothing. Then I heard a commotion – furious barking, and not just Sully’s. My heart sank. I assumed he’d got into a scrap with another dog. But how? The beach was empty.

As I walked towards the dot in the distance, I realised one of the sounds wasn’t a dog’s bark – it was more of a throaty yelp. It wasn’t until I was 20 yards away that I saw Sully was with a massive seal, three times the size of him. It was wallowing in a shallow pool of seawater and Sully was dancing around the edges, alternately charging towards it and backing off, barking wildly all the while.

It was hard to know what had happened before I arrived. Maybe the ball had landed by the seal and he’d attacked Sully when he tried to get it.

I could see a smear of blood on the seal’s coat; I couldn’t tell whose it was, as there was no obvious wound on either of them.

Then, to my horror, Sully bit the seal near one of its flippers. Appalled, I lunged forward to grab Sully by the collar and smack him on the nose. In doing so, I stumbled and fell into the icy pool right next to the seal. Up close, I was struck by its size. At least 6ft long, it was much bigger than me. It also bore an uncanny resemblance to Sully – its face had the classic bone structure of a staffy, right down to the teeth, which were at that moment sinking themselves into my elbow. Luckily, I was wearing a winter coat and they didn’t make contact with flesh.

I could feel the seal’s hot breath on my face, but I didn’t feel scared. I felt sorry for it: it was out of its comfort zone and vulnerable, defending itself the only way it knew how.

I still had hold of Sully’s collar, and as he wheeled towards me, the seal saw the opportunity to bite him. As it lumbered forwards, it rolled on top of me, pinning my legs and pushing me under the water. The sand beneath was like syrup – waterlogged and boggy – and I was sinking into it.

The situation was changing from surreal to dangerous. I was trapped under a mountain of blubber and muscle.

I realised that keeping hold of Sully’s collar was my only hope; he’s so strong and sturdy, he is as solid as a handrail. Sully somehow knew that, too, and seemed to brace himself on the firm sand at the pool’s edge to help me. Pulling hard on the collar, I managed to extricate a leg and slip out from under the seal. Staggering upright, I quickly backed away. Sully and I watched as the seal made its slow, inelegant way to the sea. I’m sure it felt as traumatised as I did.

Experience: I paid to have my daughter kidnapped Read more

Walking back up the beach, dripping wet and shivering, I tried to make sense of what had just happened. In 16 years of living here, I had never even seen a seal before, let alone been nose to nose with one; the only time I had encountered one was at the zoo. Thinking about it, the seal must have got stranded in the pool when the tide went out and decided to wait it out rather than heave itself back across the sand.

For a week or so afterwards I felt shaken up. I’ve returned to that beach several times since, but have never seen the seal again. I like to think of it in its element, perhaps exploring the Atlantic ocean, rather than wrestling a man and his dog on the Cornish coast.

• As told to Emily Cunningham

Do you have an experience to share? Email experience@theguardian.com