That's what they're speculating, that's speculation only.
( ( That ( 's ( what ( they ( 're speculating ) ) ) ) ) ( , ( that ( ( ( 's speculation ) only ) . ) ) ) )
(ROOT (S (S (NP (DT That)) (VP (VBZ 's) (SBAR (WHNP (WP what)) (S (NP (PRP they)) (VP (VBP 're) (VP (VBG speculating))))))) (, ,) (NP (DT that)) (VP (VBZ 's) (NP (NN speculation)) (ADVP (RB only))) (. .)))
That is just speculation because no one knows for sure where the plane is.
( That ( ( ( ( is just ) speculation ) ( because ( ( no one ) ( ( knows ( for sure ) ) ( where ( ( the plane ) is ) ) ) ) ) ) . ) )
(ROOT (S (NP (DT That)) (VP (VBZ is) (ADVP (RB just)) (NP (NN speculation)) (SBAR (IN because) (S (NP (DT no) (NN one)) (VP (VBZ knows) (PP (IN for) (ADJP (JJ sure))) (SBAR (WHADVP (WRB where)) (S (NP (DT the) (NN plane)) (VP (VBZ is)))))))) (. .)))