( They ( ( are ( reunited ( at ( ( K2 , ) ( ( the mountain ) ( that ( ( is ( ( second-highest but ) hardest-to-climb ) ) ( in ( the world ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (PRP They)) (VP (VBP are) (VP (VBN reunited) (PP (IN at) (NP (NP (NNP K2)) (, ,) (NP (NP (DT the) (NN mountain)) (SBAR (WHNP (WDT that)) (S (VP (VBZ is) (ADJP (JJ second-highest) (CC but) (JJ hardest-to-climb)) (PP (IN in) (NP (DT the) (NN world))))))))))) (. .)))
They are reunited at K2, the mountain that is second-highest but hardest-to-climb in the world.
(ROOT (S (NP (NNP K2)) (VP (VBZ is) (NP (NP (DT the) (JJ second) (JJS highest) (NN mountain)) (PP (IN in) (NP (DT the) (NN world)))) (, ,) (PP (IN at) (NP (IN over) (CD 28,000) (NNS feet)))) (. .)))