A thud is a bombardment ship that temporarily destroys spaces, not plots on the surface of a planet. It does this by creating a smoldering crater in a random empty space on the surface of a planet. Smoldering craters cannot be demolished until they stop smoldering, which can be anywhere from 1 to 7 days.  

Note: Spaces destroyed by the Thud do not reduce the total available number of plots available on the planet.

You can build a Thud after Munitions Lab level 3.

Playing Tip: The Thud is a ship used primarilly to harrass other empires as the after effects of a Thud hitting a planet cause no real damage.  A sufficient number of Thuds however can keep a player from building any new buildings on their planet for a few days.  In general game play though the Thud is considered to be mostly a useless ship and is primarilly used by the Diablotin.