I took a trip to South-central Manitoba.

The Cave of Swallows, also called the Cave of the Swallows (Spanish: Sótano de las Golondrinas), is an open air pit cave in the Municipality of Aquismón, San Luis Potosí, Mexico. The elliptical mouth, on a slope of karst, is 49 by 62 m wide and is undercut around all of its perimeter, widening to a room approximately 303 by 135 meters (994 by 442 ft) wide. The floor of the cave is a 333-meter (1092 ft) freefall drop from the lowest side of the opening, with a 370-meter (1,214 ft) drop from the highest side, making it the largest known cave shaft in the world, the second deepest pit in Mexico and perhaps the 11th deepest in the world.

The cave is a popular vertical caving destination. Cavers anchor their ropes on the low side, where bolts have been installed in the rock and the area is clear of obstructions. Rappelling to the floor can take up to an hour. Climbing back out may take from forty minutes to more than two hours. A person without a parachute would take almost ten seconds to freefall from the mouth to the floor, hence the pit is also popular with extreme sports enthusiasts for BASE jumping. An average-sized hot air balloon has been navigated through the 160-foot (49 m) wide opening and landed on the floor below. BASE jumpers can get out in about 10 minutes using a winch.