I apologize for cross-posting [ A really inexpensive data logger with only three components. - Exhibition / Gallery - Arduino Forum ]
but I didn't realize where that old thread was until moments after hitting post. This paper is more appropriate for the Science & Measurement sub (especially for those of you trying to deploy under water)
Cave Pearl Data Logger: A Flexible Arduino-Based Logging Platform for Long-Term Monitoring in Harsh Environments, Sensors 2018, 18(2), 530; doi:10.3390/s18020530
The journal is open access, so the PDF is free for all to download, and the logger base code is on GitHub: EKMallon (EKMallon) / Repositories · GitHub
Cheers!