At 6.45am beneath the towering mass of Fair Head the fourth buoy on Tommy Cecils line popped up as a ray of sun broke through and the current dropped rapidly in Rathlin Sound. As the current slackens, the line stops being dragged under and floats. When the fourth buoy appears, the current has dropped enough to begin the dive. We had been building up to this dive all week, and now had a little over 10 minutes of slack to get down the 48m of rope to explore the Santa Maria.

Our timing was perfect. The line was virtually slack as we dropped down through the dark, clear water. Dive fit, and having dived a dress rehearsal the previous day on the 32m Lochgarry, the descent took only 2 minutes. There she lay, the massive US oil tanker Santa Maria, torpedoed in 1918 while en route from Virginia, in the USA, to the Clyde.

It was dark, the sun not yet being high enough to penetrate to this depth, but the water was crystal clear. Our lamps lit the dense sponges which seemed to cover every inch of the intact stern section of the ship. We shone our lamps down and could just make out shapes on the gravel bottom 15m below. The plan was to work our way eastwards along the deck of the wreck to the rudder and the propeller.

It took a couple of moments to acclimatise to the subtle changes which happen at this depth - the silence, the thickness of the air, and the realisation that youre down here. We set off, our lamps illuminating walkways, encrusted handrails and the cavernous openings to the holds. We moved effortlessly back towards the stern. And there it was, a rare and fantastic sight: the huge bronze propeller, encrusted in sponge, each blade taller than a man, perfectly preserved by the depth, the vicious currents, and perhaps by the particular isolation of this part of the world.

Our computers indicated 11 minutes of decompression - well within the dive plan. After pausing to fawn for a few seconds over our uncollectable trophy, we began to work our way back towards the shot. The current, which had undetectably aided our outward journey, had now begun to increase as we worked our way back along the upper part of the deck, making use of deck fittings and railings to ease our progress. It wasnt a serious problem, but it did emphasise the need for minute-perfect planning on a dive of this nature. Nineteen minutes and we reached the line. Twenty, and with one final look round at the awesome wreck we began our ascent, compulsory time to surface now having reached 30 minutes.

Approaching 12m, 5 minutes later, my computer cleared off DEC 12 to indicate 2 minutes at 9m. At 6m the current had increased dramatically. The line began rattling and vibrating, large ragged, rust-coloured lionsmane jellyfish being whisked by ever more quickly. After 35 minutes we shot a delayed SMB and moments later were drifting, relaxed and comfortable in the current, with Tommy following above. I was glad of my little pony, even if only for comfort. Five final minutes of safety and up we came into the ripping current, have spent just over an hour in the water.

What a dive!