St Mary's Isle, more commonly called Conister Rock, was a notorious hazard. It crouches a few hundred metres offshore in the centre of Douglas Bay, half-submerged at high tide, partially exposed at low water - exactly the kind of obstacle that has destroyed ships since people first sailed into this harbour. The rock had belonged for many years to the Quane family. In 1832 Captain John Quane, then Attorney General of the Isle of Man, presented the rock as a gift to Sir William Hillary in his capacity as President of the Isle of Man District of what was then called the National Institution for the Preservation of Life from Shipwreck. That organisation would later become the Royal National Lifeboat Institution. Hillary did not merely accept the rock - he set about turning it into something useful.

Hillary contributed a substantial share of the construction costs himself and raised public subscriptions for the rest. The architect was John Welch, a respected local figure. The total cost came to £254 and 12 shillings - a modest sum even by the standards of the day, but enough to raise a stone-built castellated tower that has stood on its rock for nearly two centuries. The idea was simple and humane: a sailor wrecked on Conister Rock could climb the few feet up to the tower, take shelter inside its walls, and wait for the lifeboat. The tower contained provisions, including drinking water. It was not glamorous. It was not large. But it meant that being thrown onto this rock no longer had to mean dying of cold and exposure within sight of the Douglas waterfront.

The tower could not change the geology. Ships kept finding the rock anyway. In 1867 the schooner Thomas Parker was driven onto Conister in a storm. On 2 July 1930, the Isle of Man Steam Packet Company vessel RMS Mona ran ashore in broad daylight. After the Mona incident the authorities took unusual action: they painted the seaward side of the tower with whitewash, and painted the outer face of the Victoria Pier white too, hoping that brighter landmarks might help mariners place themselves more accurately in the bay. There is something quietly poignant about the persistence of the danger. The tower stood, the lifeboats answered, and still the sea kept depositing ships on this small unyielding piece of rock.

At low water on spring tides it has always been possible to walk out to Conister Rock from the Douglas shore, though the route is awkward and occasionally treacherous. During restoration work in the early 2000s, a makeshift causeway was constructed to allow construction plant onto the rock; that improved access has since enabled a series of community walks organised by Douglas Town Centre Management to raise money for the RNLI. The tower itself remains in the custodianship of the Royal National Lifeboat Institution, the organisation Hillary helped to found. For rowing boats in summer it has always been a destination of choice from the promenade. For the lifeboat service, it is something more: an early monument to the principle that even when you cannot prevent disaster, you can still prepare for it.