The portal was covered in mud until it was discovered in 1956.
1	The	the	DET	DT	Definite=Def|PronType=Art	2	det	2:det	Discourse=elaboration-additional:89->88:0|Entity=(127-object-new-cf1-2-coref
2	portal	portal	NOUN	NN	Number=Sing	4	nsubj:pass	4:nsubj:pass	Entity=127)
3	was	be	AUX	VBD	Mood=Ind|Number=Sing|Person=3|Tense=Past|VerbForm=Fin	4	aux:pass	4:aux:pass	_
4	covered	cover	VERB	VBN	Tense=Past|VerbForm=Part|Voice=Pass	0	root	0:root	_
5	in	in	ADP	IN	_	6	case	6:case	_
6	mud	mud	NOUN	NN	Number=Sing	4	obl	4:obl:in	Entity=(128-substance-new-cf2-1-sgl)
7	until	until	SCONJ	IN	_	10	mark	10:mark	Discourse=context-circumstance:90->89:0
8	it	it	PRON	PRP	Case=Nom|Gender=Neut|Number=Sing|Person=3|PronType=Prs	10	nsubj:pass	10:nsubj:pass	Entity=(127-object-giv:act-cf1-1-ana)
9	was	be	AUX	VBD	Mood=Ind|Number=Sing|Person=3|Tense=Past|VerbForm=Fin	10	aux:pass	10:aux:pass	_
10	discovered	discover	VERB	VBN	Tense=Past|VerbForm=Part|Voice=Pass	4	advcl	4:advcl:until	_
11	in	in	ADP	IN	_	12	case	12:case	_
12	1956	1956	NUM	CD	NumForm=Digit|NumType=Card	10	obl	10:obl:in	Entity=(129-time-new-cf3-1-sgl)|SpaceAfter=No|XML=<date when:::"1956"></date>
13	.	.	PUNCT	.	_	4	punct	4:punct	_
