His ship had a hole below the waterline the width of a football field.

CNN: Covering a cruise ship disaster

Apparently the string of logging tools, each one measuring something different, needs some room to take their readings so the "rat hole" has to extend below the measured area.

FORBES: Magazine Article

Water cascades down the side of the mountain, providing hikers with a refreshingly icy plunge into the swimming hole below.

BBC: The Philippines�� secret sanctuary

McMoran took over Blackbeard West some years ago and found hydrocarbons about 2500 feet below the point where Exxon Mobil gave up and poured cement down the hole to seal it.

FORBES: Where is McMoran's Missing Press Release?

When the explorer drills the ice to make a physical measurement of thickness, he will use the hole to lower a 4kg conductivity-temperature device (CTD) into the ocean below.

BBC: Arctic team targets key ice data

Employment is climbing out of a far deeper hole now than in 2004, and there's almost no chance the unemployment rate will be below 7.3% come November, said to be the historical dividing line between winning and losing incumbents.

ECONOMIST: The recovery and the election