People load their bicycles onto the public bus to cross the Bay Tunnel in Havana, Cuba, on April 8. RAMON ESPINOSA/AP

On a recent sweltering afternoon in the Cuban capital, dozens of commuters on bicycles, scooters and electric motorcycles gathered in a tidy row at the entrance of the Havana Bay Tunnel.

They were waiting for the Ciclobus, a bus specially fitted to take people — and their rides — through the underwater tunnel linking Old Havana to the eastern side of the island.

The diesel-powered bus can accommodate around 60 travelers and their vehicles, making enough trips to transport more than 2,000 people per day. It features a front-seating section, but half its metallic frame is an open bay for cargo. Riders enter via a specialized ramp and stay with their vehicles for the duration of the trip, holding onto wall-mounted grab bars for balance. Bicycles, motorcycles and scooters are not allowed in the tunnel.

While the Ciclobus is not new, it has never been as popular — and essential — as Cuba navigates its most severe energy crisis in decades.