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My son was thrilled when he dropped the TNT into the hole and the mountain actually fell apart to reveal the hidden tunnel. Sending his train through the blast tunnel right after that kept him entertained for a solid hour.

The mechanism is so simple that my four-year-old can trigger the blast himself, and he squeals every time the tunnel appears. It's sturdy enough to survive the repeated "explosions" he sets off, which is a huge win for me.

My son loves setting up the TNT and watching the mountain explode to reveal the tunnel, though the plastic pieces don't always snap back together perfectly after multiple blasts. The train does speed through nicely once you get the track aligned just right.