Menger sponge to five iterations.
<a href="//commons.wikimedia.org/wiki/User:Datumizer" title="User:Datumizer">Datumizer</a>
CC BY-SA 4.0
<span class="int-own-work" lang="en">Own work</span>. <a rel="nofollow" class="external text" href="https://github.com/mjhorvath/mike_illustrations">Source code</a>.