Grammar JLPT Level N4 - Lesson 29

In its citation usage, the case marking particleと is often used with verbs in the passive form. Two of these regularly occurring verbs are 考(かんが)えられている 'to be considered', and 思(おも)われている 'to be thought'.
This grammar point is used after a phrase, and translates as 'it is considered that (A)', or 'it is thought that (A)'. It is generally used to highlight a commonly held opinion that most people agree on (not your personal opinion).

• 「最初(さいしょ)はグー」と最初(さいしょ)に言(い)ったの志村(しむら)けんだと考(かんが)えられている。
  Shimura Ken is considered to be the first person who said 'saisho wa gu'. (A phrase said before playing rock, paper, scissors, in Japan)
• このトンネルは日本(にほん)で一番(いちばん)呪(のろ)われているトンネルだと思(おも)われている。
  This tunnel is thought to be one of the most cursed tunnels in Japan.

The primary reason that these constructions are used in their displacement (passive) form, is to show that the thought itself is not something that is related to the speaker's own opinions.
However, people will often use this form as a way to strengthen their own opinions, in a similar way to how English speakers would say 'everyone knows that (A)'.

• カバは世界(せかい)で最(もっと)も危(あぶ)ない動物(どうぶつ)だと考(かんが)えられている。
  Everyone knows that hippos are the most dangerous animal in the world.