A thunder egg is a type of rock formation that is formed when gas bubbles in molten lava become trapped in sedimentary rock. The gas bubbles form hollow cavities in the rock, which are then filled with agate, quartz, or other minerals. The term 'thunder egg' comes from the sound made when the rock is cracked open. The crossword solution for the term 'thunder egg' is geode, which is a type of rock formation that has a hollow cavity lined with crystals.