とはThat..., (Surprised) that..., (Never expected) that..., (Shocked) that...

とは is an emphatic structure that is regularly seen at the end of sentences, and will imply that the statement before it is shocking, appalling, or unexpected. In English, it translates very similarly as 'to think that (A)' in that it expresses a strong emotional response without expressly saying what that exact response is.

とは is a combination of the case-marking particle と in its quotation use, and the adverbial particle は. In this way, it is just marking the whole statement of (A) as what is being highlighted as the topic, without actually stating anything else about it.

とは may sometimes be seen following など or similar structures, and in these cases will just imply that 'something like (A)' is what is shocking.

Caution

Although it is common for とは to come at the end of a sentence with the actual feeling word omitted, the speaker may expressly state their reaction afterwards.