The testing and deployment was extended into the underground interconnecting tubes between the four downtown stations and further.

A major problem with such designs is that the underground tubes are subject to contamination and difficult to keep clean.

Actual CoP of a geothermal system which includes the power required to circulate the fluid through the underground tubes can be lower than 2.5.

And once they got to Sarney, they'd have to get down into the underground tube to get to Aridor.

"Maybe three days, no longer," said Croft, as he marched quietly in the underground tube, holding one of the few remaining torches.

When Croft woke again he still was on the dirt road outside of the underground tube.

They walked to a staging area between the settlement and the entrance to the underground tube from Concord.

"And then, of course, there's the underground tube."

In 1928, Theodore Pratt wrote about the system of underground pneumatic tubes that was then used to dispatch mail around the city.

The underground tube reverberated with a tremendous explosion that sounded like a mighty cannonade.