The cavity keeps a fairly uniform height, approximately between three and four metres. The widths are somewhat more variable, but overall the whole cavity is comfortable to traverse. The cavity was heavily modified by the works: widening was necessary and earth was dumped, so it is difficult to know what the original morphology was like.

In some places, the floor is concretioned by water trickles. Elsewhere, the floor consists of a dark-coloured sediment, a consequence of the accumulation of washed-in soot. Likewise, the area of the access stairs and the first metres of the cavity have black walls from the soot accumulated when steam trains ran.

Along the whole length of the cave runs a small watercourse, until it reaches the final semicircular arch channel, where it pools.

Access is via an opening in the left wall of the tunnel, about 2 m wide by 2.7 m high. Steps follow that place us at the start of the cave, all of it descending, until reaching the lower part, where it ends in a kind of channel with a half-arch vault where the water flowing through the cavity accumulates. The total route from the railway tunnel would be about 90 metres according to the survey that appears in the book El Túnel de Argentera, or 58 metres according to the GES-CMB survey.

Along the whole route we see the various works that were carried out to consolidate the cavity, especially the large number of columns present so that the ceiling does not collapse. If we have not miscounted, we can find:

• 42 columns
• 6 buttresses to support the walls
• 2 retaining walls on the south side, in order to secure the abutments of the tunnel lining.
• The channel with a half-arch vault, which is 22 m long.
• The access stair and the walls that bound it.