tunnel2 past tense and past participle tunnelled, present participle tunnelling British English, past tense and past participle tunneled, present participle tunneling American English [intransitive always + adverb/preposition, transitive]
to dig a long passage under the ground
They were tunnelling into the mountainside.
tunnel your way under/through etc
The prisoners tunneled their way under the fence.
if insects tunnel into something, they make holes in it
The grubs tunnel into the wood.