When Balmorhea pool threw a party for its grand opening over two days in June 1936, celebrants took part in all kinds of activities. A poster from the event breathlessly lists them: a bathing-girl revue, swimming competitions, something called fancy diving, a free barbecue lunch, a baseball game, speedboat races on nearby Lake Balmorhea, and not one but two big dances.That sounds like a lot of action. Honestly, I prefer my visits to Balmorhea—both the vast, gin-clear pool (which famously ranks among the world’s largest spring-fed pools) and the West Texas town you pass through to get there—on the sleepier side, which is why I like to go in winter. The state park packs people in on sultry summer days, but, like cold-blooded mosquitoes, all…