There is a version of this crossing that unfolds gradually — altitude gained politely and in stages over many days. This is not quite that version either. A day and a half acclimatising in San Pedro de Atacama helps, but the body will still have gone from sea level to above 4,400 metres (~14,400 ft) in under three days. Today is the hardest step of that climb. Take it slowly.
Provisions, Fuel, and Cash
Before leaving San Pedro, prepare deliberately. Once across the border there are no shops, no ATMs, and no reliable opportunities to resupply for several days.
Carry three days' worth of lunches, plus snacks that can be eaten cold: sandwiches, empanadas, bread, hard cheese, nuts, chocolate, and energy bars all travel well. Plan to carry 35 litres of bottled water at departure — enough for two people for four days in extremely dry air. Appetite fades before energy needs do at altitude, and dehydration arrives quietly.
Exchange and carry 2,000 bolivianos in cash for two people before departure. Cards are not accepted inside the reserve, and there are no ATMs beyond San Pedro.
Fuel requires equal forethought. Expect the full crossing to Uyuni to require approximately 120 litres. Leave San Pedro with a full 70-litre tank and a full 20-litre jerry can. This is sufficient to reach San Cristóbal, where fuel is typically available from drums and serves as the normal refuelling point before the final leg to Uyuni. Do not assume fuel is available anywhere earlier along the route.
Dinners and breakfasts are normally provided at the refugios and lodges along this stretch, including at the Termas de Polques. Lunch, however, is always your responsibility.