Resin bubble on <i><a href="//commons.wikimedia.org/wiki/Pinus_radiata" title="Pinus radiata">Pinus radiata</a></i> tree stump.  Drops of resin ooze from the stump in hot weather, sometimes the drops get blown up into bubbles from gas given off.  The bubbles then collapse again when they cool down.  A slight film of moisture has condensed in a small patch in this one.
<a href="//commons.wikimedia.org/wiki/User:Tony_Wills" title="User:Tony Wills">Tony Wills</a>
CC BY 2.5
<span class="int-own-work" lang="en">Own work</span>