While we're talking about cool boring technologies... I once heard that Seoul's Bus Terminal Station, line 9, was a particularly difficult case. Line 9 had to

While we're talking about cool boring technologies... I once heard that Seoul's Bus Terminal Station, line 9, was a particularly difficult case. Line 9 had to go beneath existing line 3, but it couldn't go too deep because it (obviously) needed to connect to the rest of the line. In the end they thrust a group of horizontal steel pipes to stabilize the soil and dug beneath it - and those pipes were 15 cm(!) below existing line 3.