Do you wish everything were flat? If you get a drawing file which you attach as an external file, and then you object snap to an endpoint or any other snap, you might catch some z coordinates along the way. Many people ask how to flatten objects, so is the answer:

Go to Options > Drafting and Opt for "Replace Z value with current elevation." (Autodesk needs to be a bit more explicit and replace "current elevation" with "current UCS.") So any Z=0 UCS counts. From now on all z values will be filtered to 0’.

Go to Select All > Properties and change the values manually for each object: Start and end z for lines, center z for arcs and circles, elevation for polylines, etc.

Dimensions are the most difficult to flatten because they have to be moved from their actual position to their reflected position at the UCS plane. If objects are flattened and your dimensions are associative, turn on the Annotation Monitor to find out whether or not your dimensions are associated. When reassociated, they’ll move to the Flatten Objects plane.       

Notes from Cadalyst tip reviewer R.K. McSwain: Danny brings up some good points regarding keeping new objects flat (at an elevation of 0.0) and moving existing objects to the zero elevation. You may be familiar with the Flatten command, but if you chose this method, use it with caution because it can do some unexpected things. Danny’s methods listed here give you predictable results. See also a couple of related CAD tips, #3903 and #4189.