Phase cloaking

The interphase cloaking device was an advanced type of cloaking device that allows a starship to travel through solid matter and high-energy fields. A molecular phase inverter moves the ship out of phase with the spacetime continuum.

In 2358, a branch of Starfleet Intelligence illegally developed such a device in direct violation of the Treaty of Algeron, which explicitly banned the Federation from developing cloaking technology.

The illegal actions and the fact that it was refered to as "a secret division of Starfleet Intelligence" implies that the 'branch' was Section 31.

A test aboard the USS Pegasus was initially successful, but the failure to maintain a steady temperature of the plasma intercoolers onboard the ship led to an explosion in engineering and later the crew, angry at the breach of the treaty, mutinied against the Captain, Erik Pressman. Starfleet Intelligence attempted to cover the incident up, but the truth was revealed in 2370 when Pressman attempted to recover the device.

Several other races have either experimented with, or utilize interphasing technology: