Where the Thames path draws level with the iconic towers of Canary Wharf on the south side of the river at Rotherhithe, we climb up and over the tide wall, then descend steep slippery wooden steps down onto an empty beach to find a place, to put the microphones. The tide is going out. Lazy waves lap and wash over the wet claggy mud. Flocks of squawking gulls scavenge along the shoreline. The air is humming with a city rumble. A vast panoramic vibration, silky, wide, like hearing the sky in sound. This area is a beating heart of global business, yet from this beach it's an astonishingly peaceful, even tranquil place. We are mid-way through another lockdown. A lone siren wavers along a distant road. Flagpoles rattle in a gentle breeze. A floating landing stage nearby rises and falls on the swell. Each time it knocks against its moorings in deep reverberant cluncks. It swings to and fro, like a slow pendulum. A tug boat gradually approaches from the west, then passes, ploughing its way east on the out-going tide. It's v-shaped bow wave rolls heavily towards the banks, then breaks past the microphones in surges of white wash. The gulls bob and leap.