Abstract
Conventional wisdom among cave divers is that submerged caves in aquifers, such as in Florida or the Yucatan, are unstable due to their ever-growing size from limestone dissolution in water. Cave divers occasionally noted partial cave collapses occurring while they were in the cave, attributing this to their unintentional (and frowned upon) physical contact with the cave walls or the aforementioned “natural” instability of the cave. Here, we suggest that these cave collapses do not necessarily result from cave instability or contacts with walls, but rather from divers bubbles rising to the ceiling and reducing the buoyancy acting on isolated ceiling rocks. Using familiar theories for the strength of flat and arched (un-cracked) beams, we first show that the flat ceiling of a submerged limestone cave can have a horizontal expanse of 63 meters. This is much broader than that of most submerged Florida caves (~ 10 m). Similarly, we show that an arched cave roof can have a still larger expanse of 240 meters, again implying that Florida caves are structurally stable. Using familiar bubble dynamics, fluid dynamics of bubble-induced flows, and accustomed diving practices, we show that a group of 1-3 divers submerged below a loosely connected ceiling rock will quickly trigger it to fall causing a “collapse”. We then present a set of qualitative laboratory experiments illustrating such a collapse in a circular laboratory cave (i.e., a cave with a circular cross section), with concave and convex ceilings. In these experiments, a metal ball represented the rock (attached to the cave ceiling with a magnet), and the bubbles were produced using a syringe located at the cave floor.
Highlights
In contrast to most aquifers in the world, the Florida aquifer is a mixture of tunnels and a continuous matrix, usually referred to as a “karst” aquifer (Figs 1 and 2)
It has been suggested recently, that the exceptional case in question of no-diver-fault was probably induced by the bubbles released by the divers, which caused resonance in the cave, leading to a collapse [1,2]. This implies the accident was due to divers actions, though, at the time, it was unknown this could cause a collapse. This is the only documented case of an accident caused by a cave collapse, there are many anecdotal collapses where rocks fell from the ceiling while cave divers were going through the cave
We find that the maximum span of the cave is incredibly large, about 2800 meters, clearly beyond the range of Florida caves
Summary
In contrast to most aquifers in the world (consisting of a continuous and relatively uniform porous media), the Florida aquifer is a mixture of tunnels and a continuous matrix, usually referred to as a “karst” aquifer (Figs 1 and 2). It has been suggested recently, that the exceptional case in question of no-diver-fault was probably induced by the bubbles released by the divers, which caused resonance in the cave, leading to a collapse [1,2]. Even without resonance, the gas released by the divers can cause a collapse (by lowering the buoyancy that forces the ceiling upward).
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