As Chris says, it's Cueva San Marcos.
When we were there last year there were cliff stabalisation works which prevented access (although some people were able to get in).
Hopefully the works have been completed but this is Spain...
Lurker, if it's Lava Tubes you want then you ought to locate in the North West, Icod, San Marcos, Los Silos, Buena Vista, etc. As I have already pointed out, most of the tubes are in the area around Icod. It takes a long time to get around the interior of the island so it's handy to be relatively near. There is lots of relatively cheap accomodation around the area in April.
I can pass on more info if you want...
Les