mujoco

<mujoco  model="collide boxes">
  <option timestep="1e-3" gravity="-1 1 -10" solver="CG" iterations="10">
    <flag multiccd="enable"/>
  </option>

  <visual>
    <scale forcewidth="0.02" contactwidth="0.05" contactheight="0.05"/>
    <map force="0.01"/>
  </visual>

  <default>
    <geom condim="1" type="box" solref="2e-3 1"/>
    <default class="free">
      <geom size=".025 .025 .025"/>
    </default>
    <default class="wall">
      <geom size=".5 .5 .1" rgba=".5 .5 .8 1"/>
    </default>
  </default>

  <worldbody>
    <light pos="1 -1 1" dir="-1 1 -1"/>
    <geom class="wall"/>
    <geom class="wall" pos="-.6 0 .4" euler="0 90 0"/>
    <geom class="wall" pos="-.2 .6 .4" euler="90 0 0"/>

    <replicate count="5" offset=".06 0 0">
      <replicate count="5" offset="0 .06 0">
        <replicate count="4" offset="0 0 .06">
          <body pos="-.1 -.2 0.15">
            <freejoint/>
            <geom class="free"/>
          </body>
        </replicate>
      </replicate>
    </replicate>
  </worldbody>
</mujoco>