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had been tunneling
  • past perfect progressive of tunnel.

Example Sentences

Examples are provided to illustrate real-world usage of words in context. Any opinions expressed do not reflect the views of Dictionary.com.

The seismic shaking could mean that magma had been tunneling underground for up to a decade before the volcanic islands appeared, the researchers said.

From Scientific American • May 30, 2015

As early as Day One of the hostage standoff, diggers had been tunneling underneath the theater in preparation for an assault.

From Time Magazine Archive