It's dark. I'm in the belly of a troop carrier, 36 metres below the surface of the water. And my oxygen tank is stuck.
I frantically reach a hand over my shoulders but the tank is where it should be.
"Relax," I tell myself.
The only sound I can hear is my own breathing, which wheezes deeply like Darth Vader.
I feel around my waist, to the left first and then the right - and then I find it. The mouthpiece of my regulator has caught the edge of the hole in the ship's wall I'm attempting to swim through.
I lift it up and it easily comes loose. I breath bubbles of relief.
I'm scuba diving on SS President Coolidge, off the coast of the island Espiritu Santo in Vanuatu.
This is my second wreck dive and I'm hooked.
The Coolidge, as it's referred to, is one of Vanuatu's best and most popular dive sites.
It began life in 1931 as a luxury ocean liner but was used as a troop carrier by the US military in 1941 and 1942.
Her demise on October 26, 1942, came quickly after she hit two underwater American mines on the seabed not far from the shore of Espiritu Santo - the largest US military base after Pearl Harbour during World War II.
The captain knew immediately the ship was in trouble, and deliberately ran aground on a reef, ordering to shore the more than 5000 troops on board. All but two people survived.
Since then, she has remained beneath the water, blossoming into a fantastic artificial reef.
"This is the world's largest accessible wreck," says Australian Paul White, owner of Aore Adventure Sports and Lodge.
Paul and local dive master Alfred Niko pick me up mid-morning from Aore Island Resort, on a small island next to Santo and not far from the Coolidge.
I'm given a thorough briefing on the safety features of the dive boat before we head to the wreck. Once moored, Alfred talks me through the dive site, the route we'll take and what we can expect to see.
Alfred has been scuba diving for 24 years and has done a whopping 14,000 dives on the Coolidge. I instantly feel comfortable under his guidance.
"It's fun," he says. "When you're happy, I'm happy. Let's go."
And with that, it's gear on and off the side of the boat we drop.
The bow of the ship is 18 metres below the water's surface. A shallow dive on the Coolidge is considered 30-35 metres, while the deepest you can go is about 65 metres.
As we descend over the top and down the side of the ship, a hawksbill turtle gracefully swims past.
The sheer size of the Coolidge is breathtaking - she's around 200 metres long, with a beam that's 27 metres wide.
Down one side we come to a stop as Alfred points at a gaping hole. He pulls out an underwater notepad and pen, and writes: "the elevator". It's astonishing, with coral-coated elevator cables spewing into the sea.
We enter the belly of the ship. It's dark and yet this magical filtered sunlight shines through rusted portholes far above us.
In some rooms, all I can see is the glow of Alfred's underwater torch and his fins.
I follow him through small rectangular cut-outs in walls, swimming sideways and occasionally becoming stuck.
This is my most challenging dive, yet I feel the most comfortable I ever have scuba diving. I could spend hours exploring the curves of this steel body.
Alfred gives me a thumbs up - the universal dive signal for "everything OK?" I return the signal and follow him out of the ship's belly, and into a cavernous graveyard.
It's lighter here, so I can make out not tombstones but the fragmented bodies of military trucks. There are gigantic tyres and wheel arches covered by barnacles and living coral. I can make out the tread in the tyres and a large steering wheel as small tropical fish flit in front of my mask. Visibility is good.
Alfred zeros in on tiny clams with red frilly lips. Under the glow of his torch their lips flicker with an electric current. It's the same with tiny flashlight fish he points out.
During our dive, we enter a room with shelves still holding medicine bottles, some with white penicillin powder.
Elsewhere, we see aircraft fuel tanks and a cooking pot, while at another point, we come across two gun mounts. Collected in one is a neat pile of ammunition shells longer than my arms. They resemble oversized bullets and Alfred delicately picks one up and pretends to fire it like a gun to help me understand what I'm looking at.
It's amazing how much is still down here and how accessible it is to tourists.
As we make our way back to the surface, the darkness disappears and a soft aqua sea envelops us.
Small black, red and yellow fish fill the water as we pass the rising bubbles of other divers deep below.
I think to myself how happy they'll be when they enter the belly of this beautiful sleeping giant.
IF YOU GO
GETTING THERE: See airvanuatu.com.
STAYING THERE: Aore Island Resort is a great place to base yourself, as it's peaceful but still close to many activities (aoreresort.com). Prices start at about $190 a night for a studio gardenview bungalow (based on a seven-night stay and including continental breakfast).
PLAYING THERE: Aore Adventure Sports and Lodge is based on Aore Island, just a quick boat ride from SS President Coolidge (aoreadventures.com). It is a PADI Dive Centre and also operates night dives on the Coolidge.
Scuba diving is good all year-round in Vanuatu, however the trade winds pass through between May and August and it's cyclone season from November to April.
MORE INFORMATION: Visit vanuatu.travel.
The writer travelled as a guest of Vanuatu Tourism Office.