Wet, dripping tones.
Low and hollow, like sound bouncing off wet stone. Play single low notes and leave space between them rather than filling every beat; the gaps are most of the effect.

Deep, dripping tones echoing off wet cave stone.
No MIDI keyboard? Play with your computer keys: bottom rows are the lower octave, top rows the higher one. Hold Shift to sustain. ← → octave, ↑ ↓ semitone.
Online ambient synth
Low and hollow, like sound bouncing off wet stone. Play single low notes and leave space between them rather than filling every beat; the gaps are most of the effect.
Hall or Cathedral turns the drips into a much larger space. With a MIDI keyboard, the mod wheel opens the tone up while a note rings.
Press Record, play, then download the take as a WAV. It is made on your own device and never uploaded, like everything else here.