The Sherkston Quarry is a remarkable 27-acre, spring-fed freshwater swimming hole on the Lake Erie shore at Port Colborne, Ontario, in the heart of the Niagara region. Carved out by a century-old limestone quarrying operation and since filled with cold, startlingly clear spring water, it's now the centrepiece of Sherkston Shores, with sandy beaches around its rim, roped-off shallows for swimming, water sports and catch-and-release fishing. Reaching nearly 70 feet deep in the middle, it is a strange and beautiful piece of industrial history turned into a swimming destination.

Its most distinctive draw is underwater. The quarry is a well-known scuba diving destination across Ontario, precisely because the mining operation left its equipment behind when the water came in. Divers descending into the clear, cold depths find the remnants of the hundred-year-old operation still sitting on the quarry floor, including original mining equipment, a locomotive, and stretches of train track. There are not many places in the province where you can swim above a sunken train.

The essential thing to know before planning a visit is that this is not public water. The quarry sits inside Sherkston Shores, a 560-acre gated private resort community and one of the largest private beachfront resorts in the Niagara region, now operated as Sun Retreats Sherkston Shores. The resort does not offer public day access to the property, the beaches, or the water park. Access to the quarry comes with renting accommodations directly from the resort, which also includes the private beaches, the water park, mini-golf and the FunPlex. In practice, that makes the quarry a destination for resort guests rather than a spot you can simply drive up to and swim.

For those staying at the resort, it's a genuinely good amenity. The shallow, roped shores make it manageable for families and swimmers of mixed ability, the sandy beach edges are pleasant for a lazy afternoon, and the deep, clear middle keeps divers and stronger swimmers happy. Golf-cart paths link the quarry to the rest of the sprawling property, along with restaurants, pools and hundreds of private cottage and trailer rentals.

Being spring-fed, the quarry runs cold, and depth drops away quickly outside the roped shallows, which is worth respecting even for confident swimmers. Summer is the season, with the resort geared to the warm months and the water at its most inviting from July into early September.

Come with sun protection, water shoes for the quarry's rocky edges, and a booking, since that's the price of admission. If you're specifically after the diving, arrange it through the resort ahead of time. For a swimming hole with a locomotive on the bottom, it's a memorable and genuinely unusual corner of Niagara.