Chart No. 1 · K 13
Dangerous underwater rock, depth unknown
Dangers
A plain cross inside a dotted circle is a sunken rock of uncertain depth, dangerous to surface craft.
What it means
A simple cross (+) is an underwater rock of unknown or uncertain depth. The dotted line around it is the danger circle, drawn around isolated hazards dangerous to surface navigation.
Why it matters
'Depth unknown' means nobody has measured how little water is over it — assume the worst. The dotted danger circle is the chart's way of saying: keep outside this line.