This dataset consists of geophysical and underway measurements collected on FS Meteor Cruise M115. Swath bathymetry data were collected using a ship-fitted Kongsberg Simrad EM122 multi-beam deep ocean echo sounder, with data collected during the cruise along specific profiles with the aim of filling gaps in existing coverage acquired by RRS James Cook cruise JC044. A total of 170 ocean-bottom seismograph deployments were made at various stations and depths throughout the duration of the cruise, of which 55 were National Environmental Research Council funded. Gravity data were acquired port-to-port using a Micro-G/LaCoste-Romberg air-sea gravimeter (S-40) mounted on a gyro-stabilised platform, which ran throughout the duration of the cruise. Magnetic and meteorological data were also collected. The cruise ran from 01 April to 28 April 2015 from Kingston, Jamaica to Pointe-a-Pitre, Guadeloupe, collecting data within the Cayman Trough. This cruise formed the field component of NERC Discovery Science project 'Crustal accretion and transform margin evolution at ultraslow spreading rates', which ran between March 2015 and June 2018. Its aims were: 1) to study the structure and lithology of the crust at the Mt Dent oceanic core complex (OCC) on the Mid‐Cayman Spreading Center (MCSC) and determine the relationship between this and the adjacent volcanic domain that also hosts hydrothermal vents; 2) to investigate how the crust changes as it cools and ages as it spreads away from the ridge axis. The Discovery Science project was composed of Standard Grant reference NE/K011162/1. The project was funded from 23 March 2015 to 30 June 2018, and was led by Professor Christine Peirce (Durham University, Earth Sciences). Data have been received by the British Oceanographic Data Centre (BODC), archived, and are available on request from the BODC Enquiries team.