Although the information regarding advance tickets no longer being an option for Grotte de Gaume is given in several postings on this forum and in some of Rick's updates, I thought I would provide the reply to an email I sent inquiring about advance tickets. Advance tickets have not been available since Jan 2018, but I found that their website is not clear about this since they indicate that 26 advance tickets are available. Apparently, there are 52 seats available and, depending on the season, people might have to occupy a seat by 6 am. Every ticket holder must be sitting in a seat (no saving seats).
The following is straight from the horse's mouth (so to speak...and I'm not talking about Mr. Ed)
"Hello,
The good conservation of this prestigious site and the maintenance of its opening to the public are directly related to the number of visitors. It is therefore important, depending on weather conditions, to modulate the limitation.
Reservations for 2018 will no longer be effective.
We hope, by this information, to have answered your questions. We thank you for your kind understanding that contributes to the preservation of this exceptional cultural heritage.
Ticket sales will be at 9.30 am at the ticket office in Font de Gaume for each day, subject to availability"