In 1999, archaeologists opened up two tunnels that allow visitors to get a glimpse of pre-existing structures below the visible surface structures. The first, Rosalila, is very short and takes only a few visitors at a time. The famous temple is only barely exposed, and behind thick glass. The other tunnel, Los Jaguares, running along the foundations of Temple 22, was originally 700m in length, but a large section has been closed, reducing it to about 80m.
The Los Jaguares Tunnel exits on the outside of the main site, so you must walk around the base and rear of the main site to get back in again. While fascinating, it’s hard to justify the L330 extra you pay to get in.