海かい底ていトンネル

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Meanings

1

noun

undersea tunnel

A tunnel built under the seabed, such as the Seikan Tunnel or the Channel Tunnel.

底bottom, sole, depth

トンネル
Generic 'tunnel'; 海底トンネル specifically refers to a tunnel under the sea.
海かい底てい
Means 'seabed' or 'ocean floor'; 海底トンネル is a tunnel built on or under the seabed.

Compound of 海底 (seabed) and トンネル (tunnel), a straightforward modern term.