Look for SYSTEM rollback segment(which can not be taken offline by the way). Here are some notes from oracle manuals:

Note: No data can be inserted into any tablespace until the
current instance has acquired at least two rollback segments
(including the SYSTEM rollback segment).

When a database is created, a single rollback segment named SYSTEM is
created in the SYSTEM tablespace. You can create any objects in non-SYSTEM
tablespaces, but you cannot write to them until you have created and brought
online at least one additional rollback segment in a non-SYSTEM tablespace (for
non-SYSTEM objects).


An instance always acquires the SYSTEM rollback segment in addition to any other
rollback segments it needs. However, if there are multiple rollback segments, Oracle
tries to use the SYSTEM rollback segment only for special system transactions and
distributes user transactions among other rollback segments. If there are too many
transactions for the non-SYSTEM rollback segments, Oracle uses the SYSTEM
segment.