What is the 'halo effect'?

What is the halo effect and does it exist?

This is a debate we see in metal detecting quite often. The halo effect is the idea that an object that has been in the ground for a long (extended) period of time is easier to detect than one that is freshly dropped due to a metallic halo, from rust (oxidation), which has formed in the soil around the object. While this can happen, this is not always the cause of the halo effect. The halo effect can occur for two reasons and sometimes they happen together 1) because the ground around the object is more compact and condensed which allows one to easier to detect the target and 2) The item can rust around the object which can cause a greater field of detection. Either or both of these instances can cause what we perceive as the halo effect. Much of this depends on the object, its environment, and how long it's been in the ground.