The Salt-Stained Map

A cartographer's map links three coastal ports. Walk it and learn what she found.

Requirements

Examine the map at Cropshore
The clerk lays a salt-stained chart on the desk. 'The cartographer who drew this washed up at Shadeshore last winter. The map marks three ports and three numbers. We don't know what the numbers mean.'
Match the first number at the seaside
The number under Cropshore is sixty-one. An old netter looks up from his catch. 'Sixty-one fathoms is the deep trench a mile out. We don't fish it. Nothing comes up alive that long underwater.'
Match the second number at Woodwater
Woodwater's number is eighty-four. The harbourmistress squints. 'Eighty-four fathoms is the shelf where the bottom drops. The cartographer hired a boat from us last summer. Asked to be lowered on a line. We didn't ask why.'
Match the third number at Shadeshore
Shadeshore's number is one hundred twelve. The dock hand laughs once and stops. 'Nothing reaches a hundred twelve fathoms. The trench keeps going. She would have known that. She was charting the bottom.'
Search the cartographer's last camp
Her camp is still under the den. A second chart is buried in oilcloth. The three numbers aren't depths. They're distances along a single line, from a fixed point west of the coast. Three ports on one chain.
Bring the second chart to the clerk
The clerk unfolds the second chart and pales. 'The fixed point is open sea. There's nothing there on any map we have. She found something below the chain three ports keep stepping over. We'll need to keep this quiet.'