▁reaching ▁the ▁down stream ▁end ▁of ▁the ▁cave , ▁they ▁discovered ▁that ▁water ▁levels ▁were ▁now ▁low ▁enough ▁to ▁see ▁a ▁series ▁of ▁low ▁arch es ▁above ▁the ▁water ▁surface . ▁By ▁anch oring ▁a ▁floating ▁cand le ▁part - way ▁through ▁the ▁passage , ▁exit ing ▁the ▁cave ▁and ▁returning ▁to ▁the ▁end ▁of ▁the ▁Grand ▁Gallery ▁in ▁Mar ble ▁Arch ▁C ave , ▁the ▁ca vers ▁confirmed ▁that ▁the ▁pass ages ▁were ▁connected ; ▁subsequently , ▁two ▁of ▁the ▁party ▁sw am ▁through ▁to ▁make ▁the ▁first ▁through - tri p ▁between ▁Mar ble ▁Arch ▁C ave ▁and ▁Lower ▁C rad le ▁H ole ▁C ave . ▁▁ 1 9 6 6 – 1 9 6 7 : ▁Sk reen ▁Hill ▁ 2 , ▁ 3 ▁and ▁Leg nab rock y ▁Way ▁ ▁By ▁the ▁mid - 1 9 6 0 s ▁a ▁number ▁of ▁adv anc ements ▁had ▁been ▁made ▁in ▁cave ▁div ing , ▁by ▁which ▁method ▁spe le olog ists ▁had ▁extended ▁their ▁expl or ations ▁into ▁c aves ▁beyond ▁the ▁sum ps ▁that ▁would ▁normally ▁have ▁halt ed ▁progress . ▁In ▁December ▁ 1 9 6 6 ▁divers ▁Dave ▁Co ble y ▁and ▁Mike ▁Bo on ▁made ▁prepar ations ▁to ▁d ive ▁S ump ▁ 1 ▁in ▁Sk reen ▁Hill ▁passage . ▁Before ▁making ▁the ▁d ive ▁however , ▁they ▁investig ated ▁a ▁small ▁dry ▁passage ▁leading ▁off ▁on ▁the ▁left ▁bank ▁of ▁the ▁lake , ▁finding ▁it ▁to ▁be ▁blocked ▁at ▁the ▁end ▁by ▁un stable