Having no bottom.
The vast swamps of Southern Ontario proved a grim nightmare to the construction gangs. Treacherous and seemingly bottomless, the swamps swallowed thousands of tons of timber and debris, yet still afforded no firm surface on which the track could be laid.
Extremely deep.
From Middle English botemles, botmeles, equivalent to bottom + -less. Cognate with West Frisian boaiemleas (“bottomless”), Dutch bodemloos (“bottomless”), German Low German boddenloos (“bottomless”), ...
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