Marathon swimming at the Tokyo Olympics is as much a test of mental endurance as it is physical conditioning over a 10-kilometer course. The outcome is often decided by weather and water conditions, as well as strategy and a furious sprint to the finish.

A blazing sun. Drenching humidity. Choppy waters. Gusting winds. Two dozen bodies diving in together for two hours of swimming mixed with a stray elbow here, an occasional kick there.

Marathon swimming at the Tokyo Olympics is as much a test of mental endurance as it is physical conditioning over a 10-kilometer course. The outcome is often decided by weather and water conditions, as well as strategy and a furious sprint to the finish.

“I think the 10k race will be slow at the start and maybe it will be like a 500-meter sprint at the end,” said Florian Wellbrock of Germany, the current world champion. “It will be a tough race with the high temperature of the air.”

Varying conditions appeal to marathon swimmers because of the challenges they present. But the potential for dangerous air and water temperatures had some swimmers, coaches and federations urging Olympic organizers to move the races from Tokyo Bay to a cooler location.