( ( The compression ) ( ( is ( ( the force ) ( caused ( by ( ( the ( ( tension and ) gravity ) ) ( of ( ( the ( earth 's ) ) atmosphere ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (DT The) (NN compression)) (VP (VBZ is) (NP (NP (DT the) (NN force)) (VP (VBN caused) (PP (IN by) (NP (NP (DT the) (NN tension) (CC and) (NN gravity)) (PP (IN of) (NP (NP (DT the) (NN earth) (POS 's)) (NN atmosphere)))))))) (. .)))
The compression is the force caused by the tension and gravity of the earth's atmosphere.
(ROOT (S (NP (NNP Gravity)) (VP (VBZ causes) (S (NP (NNS rocks)) (VP (TO to) (VP (VB roll) (ADVP (RB downhill)))))) (. .)))