Planning permission for this was not granted and in the later 1970s most of the remaining mine infrastructure was removed for use in the Oakeley Quarry, also owned by the Ffestiniog Slate Company. Croesor is connected to the nearby Rhosydd quarry by a tunnel that was built to aid surveying and settle boundary disputes. Apart from Chamber 1 East, a huge chamber in which the roof has collapsed, most of the accessible mine is in reasonable condition. Below the adit level, the mine is flooded and inaccessible. In spite of a series of collapses, particularly in the Rhosydd quarry, an underground journey from Croesor to Rhosydd is still possible and is considered to be a classic trip for mine explorers. It is known as the Croesor–Rhosydd Through Trip. Fixed ropes, home-made suspension bridges, zip wires, and inflatable boats have been installed which make it possible to complete the journey in comfort although not necessarily in safety. The Ffestiniog region contains five major slate veins, which are on top of a thick layer of feldspathic porphyry, called the "Glanypwll Trap". The veins are the New or Deep Vein, the Old Vein, the Small Vein, the Back Vein and the North Vein. At Croesor, the Old Vein is accessible, and dips downwards at about 27 degrees. It is around wide, at right angles to the bedding plane, and over horizontally. The main adit was relatively large, and ran for to reach an underground marshalling area. This level was designated as floor A.An incline led up from floor A to floors B to D Up, and there were a further three floors below floor A, designated as floors B to D Down. Another incline connected B and C Down to the marshalling area, and a separate incline connected floors C and D Down. Croesor is notable amongst Welsh slate quarries for being almost entirely underground, with no significant surface workings. It was rare in its extensive use of forced air ventilation instead of the more common natural ventilation used in most slate mines. It also generated relatively small external waste tips compared with the volume of finished slates produced; this was largely achieved by back-filling underground chambers with waste once they were exhausted. Output in the early years was quite low, but following Kellow's innovations, was between 5,000 and 6,000 tons per year but then declined until the quarry closed.