The biggest problem with CTF was the noise it made. Not the physical turbulence, but the spatial disturbance.
When the ghost ship Cross Tunneled into Thunder Abyss, it was found out precisely because of the spatial fluctuations.
As a result, Varian, Enigma, and Bali were chased by Thunder King himself! If not for Sovereign Ares personally stalling Thunder King, they'd have died.
It was a profound experience for Varian who didn't expect such a high degree of difficulty in the escape.
But after the incident passed, he got thinking.
'What if we can hide the spatial fluctuations? Stack that with ghost ship's stealth, we should be able to Cross Tunnel into any Abyss without being found, wouldn't that be awesome?'
Kyle agreed with him. But lamented that despite being a genius, he wouldn't be able to create such a machine without a decade of experimentation.
Then came Dr. Thomas and suddenly, the once distant dream was in hand's reach.