In article , "mark b"
says...

how did they dig those wells that deep anyway?


A circular wooden former is made, 4ft diameter. This allows a man to dig
inside.
The brick layer puts his bricks around the top of the former. The 'digger'
undercuts the former. The former with bricks sinks, more bricks are added,
more undercutting and so on.

Or they dig down a few feet (depending on how stable the soil is)
then fix a frame to the sides of the hole and build up to the
surface, then dig down a few more feet and build back up to the
existing brickwork ...