Upon Sekelj's arrival in Mexico in 1953, several alpine clubs invited him to take part in their treks. This was not unexpected, given the fact that his book « Tempestad sobre el Aconcagua » had practically become a manual for mountain climbing. He climbed Popocatépetl, Iztaccihuatl and many other volcanoes and mountains, further firming up his expertise in that arena. One of many fascinating explorations at that time was the underground crossing of the river San Heronimo, lying 14-km within the interior of mountain.