Chart No. 1 · K 40
Obstruction (Obstn)
Dangers
'Obstn' in a dotted circle marks an underwater obstruction of uncertain nature or depth — avoid it.
What it means
The abbreviation 'Obstn' inside a dotted danger circle marks an obstruction, depth unknown: something man-made or unidentified on the bottom that endangers navigation.
Why it matters
An obstruction could be anything — a lost container, pilings, machinery. With no depth given, treat it exactly like an unknown rock and stay outside the dotted line.