The fake Moody kept various things in this magical trunk which had seven locks with seven keys. Depending on which lock you unlocked, you would see different things inside. When the first lock is opened, the trunk holds spell books. With the second lock, the trunk revealed broken Sneakoscopes,quills, and an invisibility cloak. The seventh lock reveals a pit or chamber in which Barty Crouch Jr.hid the real Moody (GF35).
Like other things in the Wizarding World, the outside is smaller than the inside.