"That was a very important scientific discovery."
Translation:Ese fue un descubrimiento científico muy importante.
I think it's referring to that discovery, so the masculine form is needed. Eso is the neuter form.
Right, Marcy. But if the sentence were: That was very important, I think one has to go with Eso fue muy importante. ?Correcto?
I'm not a native speaker, but the way I see it is to first remove "muy" here, just to make it less confusing. Then we have "descubrimiento científico importante." Then this is simple Spanish word order where adjectives follow the noun. I believe you could switch the location of the words cientifico and importante if you wanted to, but then it just becomes a matter of which would be more clear to the listener.