Transmitting and receiving aren't the same problem. You don't need a resonant antenna, or a high-efficiency antenna, to receive perfectly well. You just need enough signal from the antenna to get above a preamp's noise floor, and that's not nearly as big of a challenge. I'm able to receive WWVB on 60kHz from 2500km away using an LZ1AQ amplifier with a pair of loops with an effective 1m diameter[*]. Granted, that's only LF, not ELF, but the idea is extensible.
There are three basic designs to start with: a dipole (or a monopole worked against ground) paired with a high-impedance "voltage" amplifier, a single-turn loop paired with a low-impedance "current" amplifier, and a multi-turn resonant loop or loopstick.
For a monopole, aluminum tubing is popular (you can easily buy them in 2m-3m lengths, and in diameters that will nest into each other, and secure them together using hose clamps), and for a loop, wire on a wooden or PVC frame is common. In either case the antenna isn't tuned for a particular frequency, it's just made big enough to work. The bigger you make it, the more signal you will get at lower frequencies — but the higher frequencies will also go get stronger, for the most part, and you will probably need to filter them out to avoid overloading the amp. A lowpass filter with a transition somewhere below 500kHz would be wise, to cut out MW broadcast stations that can have tens of thousands of watts of power.
The last option uses a loop with a larger number of turns — I've seen big loops with tens of turns on an open frame from 2 feet to 8 feet square, or hundreds of turns on a ferrite rod. Unlike the wideband active loop where you keep the inductance as low as possible, for the multi-turn loop you parallel the (relatively large) inductance of the loop with a tuning capacitor to get resonance at a given frequency. This naturally produces a narrower bandwidth. You use this with a high-Z preamp, or perhaps match it directly to feedline with a secondary loop. I haven't built one of these myself but there are materials on the internet; they're basically scaled-up versions of antennas used for AM reception. Scaling up a loopstick might require an unusually large and expensive ferrite.
[*]: Construction details for mine: each loop is a square with a diagonal of 52 inches, making for an enclosed area of 0.87 m^2, the same as a circle with a diameter of 1.05m. Each loop is made of four 12awg wires in parallel, with centers 1/2" apart, which makes for an inductance (and thus loop current) similar to that of 1/2" copper tubing, but easier and cheaper.