Assertion: cave's passage | /r/HasProperty | surveyed until early 1960s
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Source sentences:
text: The Croghan family suppressed the topographic element of Kämper's map, and it is not known to survive today, although the cave map portion of Kämper's work stands as a triumph of accurate cave cartography: not until the early 1960s and the advent of the modern exploration period would these passages be surveyed and mapped with greater accuracy.