Below is the list of tables from the database along with their columns and datatype. Need to generate an accurate SQL query for the question.
Only use the columns present in the respective table. Only use the columns which are required to generate the SQL query. Only use table joins if required and not otherwise.

### Tables ###
TABLE: CREATE TABLE table_1231 ( "Station" text, "Year opened" text, "Metropolitan borough [c ]" text, "Zone" real, "Servedby Route maps:- [ dead link ] [ dead link ]" text, "Station users 2006\u201307" real, "Station users 2007\u201308" real )

### Question ###
What's the biggest zone?

### Accurate SQL ###

SELECT MAX("Zone") FROM table_1231