Army of diggers get bored.
Army of diggers call in underground bulldozers to pile trough dirts and build lots of concrete timbers and entrences.
Until suddenly... They reach to Alaska and were stuck at the Asia-North America border so then they decide to build a very long bridge that connects from Alaska to Russia.
Then the bridge that connects to all parts of the continent starts to collapse due to the ground underwater not being stable to hold the bridge.
Then the parts of the bridge,cranes,underground bulldozers and cement mixers all fell into the ocean and it caused a medium tsunami.
The people who lived near to this bridge start to panic cuz they saw tsunami.