( ( A caldera ) ( ( is ( ( a depression ) ( ( ( similar ( to ( Crater Lake ) ) ) but ) ( without water ) ) ) ) . ) )
(ROOT (S (NP (DT A) (NN caldera)) (VP (VBZ is) (NP (NP (DT a) (NN depression)) (UCP (ADJP (JJ similar) (PP (TO to) (NP (NNP Crater) (NNP Lake)))) (CC but) (PP (IN without) (NP (NN water)))))) (. .)))
A caldera is a depression similar to Crater Lake but without water.
(ROOT (S (NP (NNS Craters) (CC and) (NNS rifts)) (VP (VBP become) (NP (NNS lakes)) (SBAR (WHADVP (WRB when)) (S (NP (PRP they)) (VP (VBP fill) (PP (IN with) (NP (NN water))))))) (. .)))