此资料的语音MP3在随时学习资源网（http://www.timemp3.com/）上可以下载，上面还有许多学习资料。Lesson42ModernCavemenCaveexploration,orpotholing,asithascometobeknown,isarelativelynewsport.Perhapsitisthedesireforsolitudeorthechanceofmakinganunexpecteddiscoverythatluresmendowntothedepthsoftheearth.Itisimpossibletogiveasatisfactoryexplanationforapot-holer'smotives.Forhim,caveshavethesamepeculiarfascinationwhichhighmountainshavefortheclimber.Theyarouseinstinctswhichcanonlybedimlyunderstood.ExploringreallydeepcavesisnotataskfortheSundayafternoonrambler.Suchundertakingsrequirethepreciseplanningandforesightofmilitaryoperations.Itcantakeaslongaseightdaystorigupropeladdersandtoestablishsupplybasesbeforeadescentcanbemadeintoaverydeepcave.Precautionsofthissortarenecessary,foritisimpossibletoforetelltheexactnatureofthedifficultieswhichwillconfrontthepotholer.ThedeepestknowncaveintheworldistheGouffreBergernearGrenoble.Itextendstoadepthof3723feet.Thisimmensechasmhasbeenformedbyanundergroundstreamwhichhastunnelledacoursethroughaflawintherocks.TheentrancetothecaveisonaplateauintheDauphineAlps.Asitisonlysixfeetacross,itisbarelynoticeable.ThecavemightneverhavebeendiscoveredhadnottheentrancebeenspottedbythedistinguishedFrenchpotholer,Berger.Sinceitsdiscovery,ithasbecomeasortofpotholers'Everest.Thoughanumberofdescentshavebeenmade,muchofitstillremainstobeexplored.AteamofpotholersrecentlywentdowntheGouffreBerger.Afterenteringthenarrowgapontheplateau,theyclimbeddownthesteepsidesofthecaveuntiltheycametoanarrowcorridor.Theyhadtoedgetheirwayalongthis,sometimeswadingacrossshallowstreams,orswimmingacrossdeeppools.Suddenlytheycametoawaterfallwhichdroppedintoanundergroundlakeatthebottomofthecave.Theyplungedintothelake,andafterloadingtheirgearonaninflatablerubberdinghy,letthecurrentcarrythemtotheotherside.Toprotectthemselvesfromtheicywater,theyhadtowearspecialrubbersuits.Atthefarendofthelake,theycametohugepilesofrubblewhichhadbeenwashedupbythewater.Inthispartofthecave,theycouldhearaninsistentboomingsoundwhichtheyfoundwascaus