The Enterprise enters asteroid gamma 601.
Asteroid gamma 601 was an asteroid in the Devolin system. The asteroid was fairly large and had several deep chasms large enough for a starship to enter. However, the internal structure was highly unstable; phaser fire within the chasms could cause collapse. The asteroid was located in the proximity of asteroid alpha 331 and asteroid beta 671.
Sometime after 2357, the hull of the interphase cloaked USS Pegasus drifted into the Devolin system and rephased while partially embedded approximately two kilometers within the asteroid.
In 2370, Starfleet Intelligence learned that the Romulan Star Empire had possibly found the wreck of the Pegasus in the Devolin system. The USS Enterprise-D was dispatched to investigate, joined by the former commander of the Pegasus, Admiral Erik Pressman. The Enterprise competed with the Romulan warbird Terix to locate the exact wherabouts of the Pegasus, finding it first within asteroid gamma 601 The asteroid was blanketed with high levels of ionizing radiation in order to hide the ship so it could be salvaged without Romulan interference.
The Enterprise returned one day later, but the Pegasus was too deep within the asteroid to be accessed by transporter or shuttlecraft. Against Captain Jean-Luc Picard's wishes, Pressman ordered the Enterprise to enter a chasm on the asteroid which led to the Pegasus's location, the first time a starship had been taken so deeply within a planetary body. While Pressman was able to retrieve the interphase cloaking device from the Pegasus engineering section, the Romulans used their disruptors to seal the Enterprise within the asteroid, claiming that they were conducting geological experiments. Commander William T. Riker, another former member of the Pegasus crew, defied Pressman's orders and revealed the existence of the interphase cloak to Picard. It was then installed aboard the Enterprise and used to escape the asteroid's interior. (TNG: "The Pegasus")