Selecting points at bottom of a mesh only

Hi everyone,

How can I best try to select only the vertices at the bottom of a defined rhino mesh. I have applied a deconstruct mesh component to get the verticies, however my mesh has 70,000 vertices. Hence why i need a quick way of selecting only the bottom layer. Ive tried to list item by unit z vector etc but doesnt seem to be doing what I need. Any ideas? Thanks.

You can take a look at this thread from the old forum. It uses the Sandbox plugin from