Cannonball Cave is not a cave you walk into. It's one of Missouri's premier underwater cave systems, a fully submerged spring cave beneath Lake Wappapello just south of Greenville, and it is the exclusive province of trained, certified cave divers. Known to locals as Davidson Spring or Davidson Blue Spring, after the family who homesteaded the surrounding land, it was drowned when the damming of the St. Francis River created the lake in 1941, and divers only established access in the 1980s.

This must be stated plainly: Cannonball is a technical cave dive, not a recreational swim or a walk-in cave. Cave diving is among the most demanding and unforgiving activities in the outdoors, and this system in particular is one for experienced, cave-certified divers with the right training, equipment and team. The entrance alone makes the point: it's a restriction roughly two and a half feet high and twelve to fifteen feet wide, with only the centre portion usable by back-mounted divers. Organizations like the Ozark Cave Diving Alliance, who have pushed the system, have the resources and skills the place demands.

For those qualified, it is a remarkable piece of underwater geology. Past the restriction, the cave opens after about fifteen feet into a large tunnel that runs to a pit a little over 700 feet from the entrance. Its signature is the clay: there are absolutely stunning clay formations throughout, especially in the upper tunnels, with places showing fifteen or more layers of coloured clay banded in black, orange, white and grey. It is a genuinely unusual and beautiful thing, and there is little else like it in the Ozarks.

The spring is powerful. The opening discharges an enormous volume of water, with measurements ranging from 25 to 50 million gallons a day, and that flow shapes every aspect of a dive here. Water temperature sits in the mid-50s Fahrenheit year-round, and visibility swings dramatically, ranging from very low to somewhere around 40 to 50 feet depending on conditions.

For everyone else, Lake Wappapello is a fine destination in its own right, with the surrounding state park and Corps of Engineers land offering swimming, fishing, boating and camping in the rolling Ozark foothills of southeast Missouri, all without a single overhead environment.

If you are cave-certified and considering Cannonball, connect with the Missouri cave diving community first for current conditions, access arrangements and guidance. If you're not, admire the photographs. The cave has earned its reputation, and it is not a place to learn.