Complex sentence: While exploring the cave they stumbled upon a deep karst sinkhole, formed by the action of running water on limestone that revealed a tunnel about long in which they found the Altamura Man incorporated into the calcium carbonate concretions caused by water dribbling down the cave walls.
Split into simpler sentences: While exploring the cave the researchers stumbled upon a deep karst sinkhole, formed by the action of running water on limestone. The sinkhole merges into a tunnel about long in which they found the Altamura Man incorporated into the calcium carbonate concretions that had formed by water dribbling down the cave walls.